I would have to say it would be the Liberation of Manila by US forces.
Manila was declared an open city and the Imperial Japanese troops was in the process of leaving. When US officers learned that members of the Imperial Japanese Navy weren't going to leave, the US decided to use artillery and bombers to pound the Japanese. The US succeeded but they destroyed an entire city in the process. The devastation was described as almost like what happened to Warsaw.

Tens of thousands of Filipino civilians were killed in what I view was an unneccesary battle. Whole families were wiped out by Japanese bayonets and indiscriminate US artillery. Personally, when I think and read the accounts of this battle, I feel agitated.