A crime. An act of war is military in nature. This wasn't a military action. Mexican officials tipped off US officials to the act. Thus it wasn't an act of war. Simple really.
And 9-11 was just an isolated criminal act, because they didn't wear uniforms and fly a flag as they flew into the WTC.
Some people are so stupid it's not worth trying to communicate with them. I put you squarely in that category.