an act of war
1. A violent action that could initiate large-scale fighting. If we invade that country, it will be an act of war.
2. By extension, any unwelcome action that elicits an angry response. You played my video games when I wasn't home? Oh, that's an act of war, little brother!
an act of war
1. Lit. an international act of warlike violence for which war is considered a suitable response. To bomb a ship is an act of war.
2. Fig. any hostile act between two people. "You just broke my stereo!" yelled John. "That's an act of war!"