In February 1821, in the rugged mountains of southern Mexico, two former enemies stepped forward and embraced each other. Their respective troops, assembled to watch the spectacle, viewed those opposite them with suspicion. For a decade, they had fought a bitter and brutal war; sometimes in the frontal assaults and sieges of Napoleonic warfare and sometimes in hit-and-run guerrilla and counter-guerrilla warfare. Vicente Guerrero, who in 1829 would become Mexico’s first president of part-African descent, led the insurgents fighting for independence from Spain. Agustín de Iturbide led the royalist forces. Guerrero was a man of the people, at home among the rustic camps and hardened fighters of the tierra caliente, the ‘fiery lands’ of the southern coast. Iturbide was a genteel royalist officer of peninsular Spanish descent and above all, an opportunist.

It was an unlikely embrace, to say the least. Understanding why it happened, helps us to unlock the secrets of this crucial year and how it helped to create an international community permanently concerned with managing a world order. In 1821 the experience of interconnected challenges and political movements from different continents, converging into dramatic flashpoints, gave birth to what we might call world politics.

Iturbide represented a gamble on the part of some in the ruling classes of New Spain, as Mexico was then known. They reasoned that the hardscrabble rebel Guerrero could be tamed and that their aristocratic champion Iturbide would come out on top. Indeed, for a brief period, from mid-1822 to early 1823, Iturbide was nothing less than Emperor of Mexico.

This gamble of February 1821 was a reaction to the Spanish Revolt of the previous year. In January 1820, Lieutenant-Colonel Rafael del Riego, commander of a Spanish army battalion waiting to be sent to the Americas to fight the likes of Guerrero and uphold the empire, decided not to go overseas and instead started a rebellion against royal absolutism. His troops marched through Andalucia but aroused little enthusiasm. Yet Riego’s luck improved in the northern province of Galicia, where more rebels appeared, and by March rebellious troops had surrounded the royal palace in Madrid and compelled King Ferdinand VII to accept the Spanish Constitution of 1812. This classic document of liberalism had been devised during the excitement of Napoleon’s invasion of Spain and repudiated by the restored Ferdinand in 1814. It prescribed a parliamentary system with a separation of powers and freedom of speech as well as something close to universal male suffrage.

At this point, the aristocracy of New Spain preferred to part ways with the metropole, thinking that the old order would be safer without it. Accordingly, they backed Mexican independence and, after a few skirmishes with diehard Spanish troops, the new combined army entered Mexico City in triumph in September 1821. The independence of Mexico, the largest and wealthiest of Spain’s American possessions, had been preceded by and would be followed by the others. Brazil retained the old royal family but also became independent. British diplomatic recognition and loans created a new web of transatlantic relationships and a greater sense of an international community. ‘I called the New World into existence to redress the balance of the old,’ boasted George Canning, the British Foreign Secretary, in the House of Commons, in 1826. Britain and the United States vied for influence in independent Mexico; the US ambassador, Joel Poinsett, meddled in Mexican politics and vehemently denounced the British at dinners with the Mexican elite.

Riego’s army was initially set up to keep an empire intact, but contributed instead to its disintegration. It did so, moreover, in an ominous fashion because the revolt raised questions around constitutional government and national identity. This boisterous, militant entwinement of nationalism and constitutionalism would constitute the supreme challenge to the reformed old order, which had been restored and revived in the wake of the French Revolution and Napoleonic Wars. The transnational aspects of that challenge and the nature of the response to it, helped bring about the sense of an international community as is understood today.

Prince Metternich, Austria’s foreign minister, sat at the geographical and intellectual heart of what came to be known as the Concert of Europe. It consisted of periodic grand congresses, beginning in Vienna in 1814, which gathered the monarchs and statesmen of Europe to redraw its map and political order. From the start, Metternich had insisted on inserting political principles into this geographical reordering, whereby kingdoms lost and acquired provinces. He was most successful in Germany, where a police information centre kept watch on nationalist students and constitution-hungry democrats in the different states.

A pragmatic conservatism favouring a strong monarchy with an element of devolved local government and an opposition to both nationalism and representative government characterised Metternich’s politics. Sometimes this stretched to asking the princes of smaller states in Europe to issue statements in favour of strict monarchical principles which purported to come from them but had in fact been drafted in Vienna, in Metternich’s own office. Generally, he acted in harmony with Prussia and Russia, especially during the crackdown in the German states in 1819 after Karl Sand, a radical student, had murdered the playwright August Kotzebue, then acting as Russian consul-general in the Grand Duchy of Baden, for having made fun of German nationalism. But while Metternich established a reasonable degree of political and ideological control over the German Confederation, of which Austria held the presidency, he was less successful in Italy, where the Russian Tsar, Alexander I, had stopped him from establishing an equivalent League under another Austrian presidency.

Riego’s boldness inspired a whole host of constitutionalist secret societies in Europe with new hope. A copycat revolt in Naples in 1820 proclaimed the Spanish constitution of 1812 and compelled its own monarch, also called Ferdinand, to swear loyalty to it. While Ferdinand squirmed and complained privately, liberal sects like the Carbonari, mobilised and made their presence felt in the city of Naples. At first, a moderate government took over and showed some willingness to negotiate with an infuriated Metternich. The great powers of Europe, gathered at Troppau, condemned the revolt, though some more emphatically than others.

Yet Metternich wanted no negotiations with Naples. The revolutionary precedent was too destructive. He wanted the Austrian army to march in, suppress the constitutionalist government, and establish a regime under Austrian supervision. After that, the Italian League, which he hoped for, might be established after all.

The problem, once again, was the Tsar, or rather his adviser, Capodistrias, who insisted on negotiating with the moderates in Naples. The powers gathered again at Laibach, now Ljubljana, in January 1821. Ferdinand was summoned to Laibach by Metternich. Before he left Naples, the king pleaded with his national assembly to be allowed to leave to negotiate with the great powers. He pledged undying fealty to the government and the Spanish constitution of 1812. As soon as he arrived at Laibach, he repudiated his oaths, but then exasperated Metternich by spending all his time hunting and by refusing to read papers longer than one page.

As Metternich overcame diplomatic obstacles, including the Pope’s half-hearted opposition to sending an Austrian army charging down the Italian peninsula, the radicals in Naples made his life easier. They overthrew the moderate government, imprisoned its ministers and declared that they would fight the Austrians. Metternich now had the confrontation he wanted. With Prussian support, and after soothing conversations with Tsar Alexander, Austria proclaimed a restoration of order in Naples. The Austrian army brushed the Neapolitan forces aside at the battle of Rieti on 7 March and entered the capital city.

The three eastern powers, Austria, Prussia and Russia, then pioneered a new principle: they presented this action as salutary against the backdrop of a transnational ideological confrontation between royal authority and constitutionalism. This was first ideologically motivated military intervention sanctioned by an international body invested with the highest authority and in continuous session. Britain was sceptical and critical voices were heard in the House of Commons.

Metternich could afford to join Ferdinand at the hunt, if he chose. He had triumphed over the revolutionaries, the scruples of the Pope, the hesitation of the Russians, and the grumbles of the French and British. Yet on the same day that Austrian forces had routed the Neapolitans at Rieti, an officer of the Tsar’s army, Alexander Ypsilanti, crossed the River Prut dividing the Russian Empire from the Danubian Principalities – the borderlands of the Ottoman Empire – and proclaimed a revolt in the name of the Greeks and the Ottomans’ Christian subjects. He did so without the Tsar’s permission. This was revolt was a clarion call for Greek independence and set of rebellions in Greece itself, which soon gathered the sympathies of romantic onlookers all over the world, including Lord Byron, who died there in 1824. The Greek fight against Ottoman rule became a universal symbol of the struggle for freedom.

From the perspective of conservative politics, which sought to maintain stability and a balance of power, this was dangerous stuff. Metternich scrambled to prevent a war between the Ottomans and the Russians, who claimed to be the protectors of the Christian peoples inside the Ottoman Empire, a position which had been strengthened by the Treaty of Kuchuk-Kainarji in 1774. Metternich’s diplomacy now managed what became known as the ‘Eastern Question’, the gradual decline and retreat of the Ottoman Empire, trying to prevent not just major wars from breaking out, but also any triumph of the principle of revolution – the idea that peoples could overthrow legitimate governments. The stakes could not be higher.

Metternich succeeded in preventing war between the Russian and Ottoman Empires, at least for several years. In 1822 he intercepted and detained emissaries who had been sent by the Greek rebels to the Pope in the hope of persuading him to proclaim a new crusade against the Ottomans. The management of the ‘Eastern Question’, which the international community of great powers began in 1821, would continue to pose immense challenges and would become one of the causes of the First World War in 1914.

Yet Metternich’s woes were not over. The same week that Ypsilanti crossed the Prut, in March 1821, rebel officers in Piedmont, an independent kingdom in north-western Italy, seized the Citadel in Turin and demanded the Spanish Constitution of 1812 and war against Austria. Unlike the Neapolitans, the Piedmontese rebels explicitly called for the creation of a unified Italian nation.

Still at Laibach, Metternich lost no time in gaining the support of Prussia and Russia for another intervention. This time, he asked for 90,000 Russian troops to help. The idea of so many Russians marching across the Austrian empire horrified its foreign minister Stadion, Metternich’s rival, but in the end the Piedmontese revolt was defeated quickly, at Novara, in April 1821. Moreover, the Austrians made Naples and Piedmont pay occupation costs.

Yet the call for the expulsion of Austria and for Italian unity was ominous. It was the rallying cry of the Risorgimento, the movement for Italian unification, and tied constitutionalism and nationalism together more explicitly than the Neapolitans and Spaniards had done. This challenge, first fully articulated in 1821, would overthrow the aristocratic and Metternichian iteration of the international community over the course of the century. It introduced volatile democratic electoral dynamics and public sentiment more fully into international politics.

Metternich’s ingenuity and resilience in 1821 had allowed the international community, in the form of the monarchs and ministers of the great powers, to weather the storm of multiple challenges. It thereby established the principle that such a community could manage international affairs as a continuous, unbroken effort and that such management could be underpinned by detailed principles of governance. This was new. Even if the international community was largely European and its principles, insofar as there was consensus, were those of a moderate conservatism, the precedent was significant. By 1823, the conservative powers agreed to suppress the Spanish Revolt and Riego was shot. Yet his fame lived on and the Second Spanish Republic of the 1930s named its national anthem in his honour. The 1823 invasion of Spain by French royalist troops alarmed the US President, James Monroe. In his message to Congress in December he proclaimed what came to be known as the ‘Monroe Doctrine’, excluding European powers from the western hemisphere. This idea of hemispheric sovereignty was also new.

Monroe had just weathered the storm over slavery that ended with the Missouri Compromise of 1820-21. Recent research has shown just how significant his mediation was in attaining the compromise. It followed a crisis in which the union itself was threatened. It began in 1819 when the prospective statehood of Missouri came before Congress. Representative James Tallmadge of New York moved an amendment which required Missouri to exclude slavery in its state constitution as a condition for joining the union. In a fiery and dignified speech, he argued that the legitimate monarchs of Europe, adversaries of the US republican system, were laughing at the United States because of the existence of slavery. The amendment was seconded by Representative John Taylor and passed by the House. Southerners erupted in indignation, predicting that oceans of blood would result from it. ‘So be it!’ replied aroused northerners. Former President Thomas Jefferson wrote to a correspondent in 1820 that the problem of slavery had awakened him ‘like a fire bell in the night’.

According to some observers, President Monroe accepted the continued Spanish possession of Texas in the Adams-Onís Treaty of 1819, because he feared that the acquisition of this wealthy province would extend the terrain of slavery and upset the peace and balance of the union. In 1820, John Taylor offered a compromise: Missouri would be admitted as a slave state but slavery would be prohibited in any future state north of latitude 36°30’. The Virginian politician John Randolph, whose feelings about slavery were ambivalent, scoffed at those northerners whose morality only went as far as 36 degrees, 30 minutes north latitude. Many southerners claimed victory, but Jefferson expressed deep foreboding, writing to John Holmes,

A geographical line, coinciding with a marked principle, moral and political, once conceived and held up to the angry passions of men, will never be obliterated; and every new irritation will mark it deeper and deeper.

In February 1821, with Missouri still not formally admitted, northern restrictionists opened a new front. The proposed constitution of Missouri prohibited all slave emancipation in the future and banned the immigration of free blacks. The anti-slavery restrictionists chose to attack the second of these clauses because the US constitution guaranteed to ‘citizens of each state the privileges and immunities of citizens in the several states’. The southerners were enraged. Charles Pinckney of South Carolina angrily claimed that he himself had written that very clause of the constitution and that he did not have black people in mind when he wrote it.

More angry words and constitutional arguments were exchanged until Henry Clay of Kentucky intervened. He suggested that the Missouri constitution adopt anodyne wording that paralleled that of the US Constitution but which was still open to interpretation. The exhausted contenders, fearing for the union of the states, accepted the compromise. Once again, many southerners, including Pinckney, loudly claimed victory. Most northern restrictionists were dejected, yet some secretly celebrated a triumph of their own. They had forced the southerners to openly debate the rights and wrongs of slavery and its relationship to the constitution, something the southerners had desperately sought to avoid. By establishing the geographical line dreaded by Jefferson, they had opened the way to a conception of nationhood that excluded slavery. More quietly, but just as tangibly, they had given their preferences geopolitical weight by denying Texas to the South, at least for the time being. Like Mexicans and Europeans, they had grappled with the problem of nationality and constitutions and established the principle of consultative management for thorny questions, even if the compromise only postponed the reckoning with the institution of slavery.

The rise of diverse nationalisms, their entwinement with problems of constitutional government and geographical lines of demarcation, and the deft management of these challenges by the representatives of the old order, created something new: the sense of the international order as we know it. The term ‘world order’ began to be used in learned publications in in the 1820s. But beyond high politics, the sense that causes and movements in one continent resonated vividly in another, created the idea of a unified world order at an intellectual, even poetic level. As a group of white Virginia women in the 1820s enthused about the Greek rebels and engaged in sewing to aid them, a coffle of manacled slaves passed by. John Randolph said to the women, ‘Ladies, the Greeks are at your doors.’