Well, it wasn't "an act of war". It was a serious act of terrorism by a private group of conspirators.

Calling it "war" is merely rhetoric, designed to quieten critics at home, and in the process helping the conspirators acquire prestige and power, and recruit more members.

The United States is very lucky that the Oklahoma bombing didn't lead to a similar sort of rhetoric - the people around McVeigh would have been immeasurably helped if they had been presented as an organisation who had turned the USA into a state at war, in which many civil liberties were no longer a luxury that could be afforded.
By this time you very probably would have some kind of civil war taking place.