What is an act of war?

This came from a Madison friend of mine last week (yeah I am late):

After the revelation of the plot to assassinate the Saudi ambassador to the US, Representative Peter King stated such a plot is essentially a declaration of war on the US by Iran.

House Homeland Security Committee Chairman Peter King says the Iranian plot to assassinate Saudi Arabia’s ambassador and blow up the Saudi and Israeli embassies in Washington “crossed a red line,” and had it been successful “basically you are talking about an act of war.”

So the question is, if this plot is an act of war against the US, what exactly are our continued drone assassinations in Pakistan and Yemen considered? Aren’t we essentially waging war in and possibly against Pakistan and Yemen? And is the reason they only verbally complain is that they fear our military and need our foreign aid dollars? But how long can WE support continued undeclared warfare in supposed allied and sovereign nations?

H/T Paul W.