Seven Years%27 War
In early 1758, Frederick launched an invasion of Moravia, and laid siege to Olmütz (now Olomouc, Czech Republic). Following an Austrian victory at the Battle of Domstadtl that wiped out a supply convoy destined for Olmütz, Frederick broke off the siege and withdrew from Moravia. It marked the end of his final attempt to launch a major invasion of Austrian territory. East Prussia had been occupied by Russian forces over the winter and would remain under their control until 1762, although Frederick did not see the Russians as an immediate threat and instead entertained hopes of first fighting a decisive battle against Austria that would knock them out of the war.

Q: What city did Frederick lay siege to?
Olmütz