[also Argentine-Brazil War]

In 1825 war flared again over the Cisplatine Province, this time with Buenos Aires determined to annex the East Bank. The empire could little afford the troops, some of whom were recruited in Ireland and Germany, or the sixty warships needed to blockade the Río de la Plata. A loan from London bankers was expended by 1826, and Pedro had to call the General Assembly to finance the war. The blockade raised objections from the United States and Britain, and reverses on land in 1827 made it necessary to negotiate an end to the US$30 million Cisplatine War. The war at least left Uruguay independent instead of an Argentine province. In June 1828, harsh discipline and xenophobia provoked a mutiny of mercenary troops in Rio de Janeiro; the Irish were shipped home and the Germans sent to the South. The army was reduced to 15,000 members, and the antislavery Pedro, now without military muscle, faced a Parliament controlled by slaveowners and their allies.

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war against Brazil had begun in 1825. The Argentine forces were able to defeat the Brazilians on the plains of Uruguay, but the Brazilian navy blockaded the Río de la Plata and succeeded in crippling Argentine commerce. Rivadavia, unable to end the war on favourable terms, resigned in July 1827, and the national government dissolved. Leadership of the province of Buenos Aires was given to a federalist, Colonel Manuel Dorrego. Dorrego was backed by local interest groups whose political spokesman was the great landowner Juan Manuel de Rosas, who had been named commander of the rural militia. Dorrego made peace with Brazil, and in 1828 the disputed eastern province was constituted as the independent state of Uruguay. The Uruguayan lands, which Rivadavia had considered indispensable to the "national integrity" of Argentina, were never to be recovered. In December 1828 troops returning from the war overthrew Dorrego and installed General Juan Lavalle in his place; Dorrego was executed.