The Paraguay expedition (1858–1859) was an American diplomatic mission and nineteen-ship squadron ordered by President James Buchanan to South America to demand redress for certain wrongs alleged to have been done by Paraguay, and seize its capital Asunción if it was refused. The expedition was sent without an adequate investigation of the facts; most modern scholars have considered Buchanan's complaints were probably unjustified. The real cause of the misunderstandings was that neither country had employed a competent diplomatic service. Buchanan may have had an ulterior motive, such as to distract public opinion from the domestic concerns that afflicted his presidency; even more sinister explanations have been suggested; but possibly Buchanan may have meant to emulate the success of the earlier Perry Expedition, which had "opened up" isolated Japan.
At the time it was the largest naval squadron ever sent from the United States, and it caused a great impression in the Platine basin. Even so, had it come to war, the strategic position of Paraguay in the heart of South America was strong. Further, owing to shore-based administrative incompetence, the squadron had serious deficiencies. Hence the expedition has been described as "woefully inadequate" for the job and "a military bluff carried off with commendable skill". However, because of the conciliatory attitude of the U.S. diplomatic representative Judge James B. Bowlin — who to get an equitable resolution deviated from his instructions — and to the fact that Paraguay needed international friends, normal relations between the two countries were restored. Until that happened, only one ship entered Paraguayan waters, by mutual agreement.
In financial terms, the expedition cost perhaps $3 million for which Paraguay paid $9,412 for the sake of peace and quiet. Nevertheless Buchanan claimed a foreign policy success. There was disinformation, some of whose effects persist to this day.