The eldest son of Ferdinand I, Boris assumed the throne upon the abdication of his father in the wake of Bulgaria's defeat in World War I. Under the 1919 Treaty of Neuilly, Bulgaria was forced to, amongst other things, cede various territories, pay crippling war reparations, and greatly reduce the size of its military. That same year, Aleksandar Stamboliyski of the agrarian Bulgarian Agrarian National Union became prime minister. After Stamboliyski was overthrown in a coup in 1923, Boris recognized the new government of Aleksandar Tsankov, who harshly suppressed the Bulgarian Communist Party and led the nation through a brief border war with Greece. Tsankov was removed from power in 1926, and a series of prime ministers followed until 1934, when the corporatist "Zveno" movement staged a coup and outlawed all political parties. Boris opposed the "Zveno" government and overthrew them in 1935, eventually installing Georgi Kyoseivanov as prime minister. For the remainder of his reign, Boris would rule as a "de facto" absolute monarch, with his prime ministers largely submitting to his will.