| Treaty of Vienna | |
| Type: | Offensive alliance |
| Location Signed: | Vienna, Archduchy of Austria |
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The Treaty of Vienna, concluded on 27 May 1657, was an Austro - Polish alliance during the Second Northern War.[1] 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.