In 1815, Leopold moved to England. He acquired British nationality and became a Field Marshal in the British army. In 1816, he married Princess Charlotte, daughter of the Prince Regent (the future George IV), and heir to the British throne. Leopold was preparing himself for the role later played by Prince Albert, consort of Queen Victoria. (Albert and Victoria were actually his nephew and niece). Leopold's happy and promising marriage, however, ended tragically, the following year, when Princess Charlotte died in childbirth, along with her infant son. Charlotte was deeply mourned by her husband and people.
Despite this disappointment, fortune would favor Leopold. In 1830, he was offered the throne of Greece, but refused to accept. The following year, he was offered, and accepted, the Belgian crown. At the Congress of Vienna, the Belgian provinces, formerly ruled by the Austrian Habsburgs, had been joined with the Dutch provinces, in a united Kingdom of the Netherlands headed by the House of Orange. For different reasons, however, Catholics and Liberals in Belgium developed grievances against the heavy-handed and fiercely Calvinist King William I. In 1830, a pragmatic alliance of the (normally mutually hostile) Catholics and Liberals had overthrown Dutch rule. The emerging Kingdom of Belgium was now seeking a Sovereign. Prince Leopold was considered exceptionally capable, and his British connections were highly desirable. (Great Britain was one of Belgium's main supporters in her struggle for independence, and, of course, later, during World War I). Thus, although a Protestant prince, Leopold became the first King of a predominantly Catholic country. He swore his accession oath on July 21, 1831; this date has been commemorated, ever since, by the celebration of Belgium's National Day.
Soon after his accession, Leopold confronted a Dutch invasion, aiming at the re-conquest of Belgium. Leopold personally commanded his forces, setting a precedent for future Belgian kings Albert I and Leopold III. Unimpressed by his followers, Leopold remarked, with the caustic irony of the Saxe-Coburgs: "Half of the army betrayed me, the other half fled" (quoted by Queen Marie-José of Italy in her memoirs, Albert et Elisabeth de Belgique, mes parents, 1971). Nonetheless, he made himself loved by sharing the life of his troops. With the aid of the Great Powers, Belgium maintained her independence, but was compelled to make territorial and economic concessions to the Netherlands. The Treaty of London, in 1839, guaranteed Belgium's status as a sovereign and neutral state.
In 1832, Leopold married Princess Louise Marie of Orléans (1812-1850), daughter of King Louis-Philippe of France. The couple had three surviving children: Leopold (1835-1909), future King Leopold II of the Belgians, Philippe (1837-1905), Count of Flanders, father of King Albert I of the Belgians, and Charlotte (1840-1927), later Empress Carlota of Mexico. Despite the Orléans' association with liberalism, freemasonry and revolution, Louise-Marie (pictured below) was a devout Catholic. The royal children were also raised in the Catholic Faith.
Leopold devoted the rest of his reign to consolidating the young state and quietly furthering its development. He managed to maintain a balance between the Catholic and Liberal parties. In 1835, he saw the culmination of one of his long-nurtured projects: the inauguration of the Brussels-Malines railway line, the first in continental Europe. In 1842, in the face of rising industrialism in Belgium (a process he encouraged), the King attempted to introduce legislation regulating the labor of women and children. Unfortunately, however, his enlightened ideas did not find fertile ground. In 1848, a republican revolution dethroned Leopold's father-in-law, Louis-Philippe, but the upheavals spared Belgium, largely due to Leopold's skillful diplomacy. He was widely admired for his political wisdom; a number of European monarchs, including his niece, Queen Victoria of England, sought his advice. His prudence and tact won him the title "Nestor of Europe."
On December 10, 1865, after successfully establishing Belgium as a sovereign nation, and founding a new royal dynasty, Leopold died. He remained firmly Protestant to the end; despite the earnest appeals of his entourage, he refused, on his deathbed, to convert to Catholicism. He was succeeded by his son, King Leopold II of the Belgians.