1657 alliance between Austria and Poland–Lithuania
The Treaty of Vienna, concluded on 27 May 1657, was an Austro–Polish alliance during the Second Northern War. 
After Habsburg emperor Ferdinand III had agreed to enter the war on the anti-Swedish side and support the Polish king John II Casimir with 4,000 troops in the ineffective Treaty of Vienna (1656), his death in April 1657 made way for a more substantial treaty with his successor Leopold I. By this treaty, Leopold I promised to aid John II Casimir with 12,000 troops against the Swedish-Brandenburgian alliance. These troops were to be maintained at Polish expense, and crossed the Polish border in June.
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