North Korea just fired a ballistic missile directly over one of our allies... isn't that an act of war?

Usually these questions, are controlled by a specific Treaty. SEATO is not relevant because the declarations of the .U.N. are controlling.

The USA bombed Syria, is that an act of war?

It certainly is. NK has the nuke, missiles, starvation sanctions placed on it by the UN and USA, a stupid-crazy leadership, an impoverished small population, a prosperous South Korea next door, a desire to dominate South Korea and unite the two Koreas, and close relations with Russia and China. Put it all together and it doesn't spell disaster; it spells negotiations, disarmament, change of leadership, and food and assistance in exchange for that.

Write "BOOM" on a dud casing, and land it in the center of Pyongyang. Message sent, and received!

It was unarmed.

It was an act of trespass.

Want to nuke them for trespassing?

I wonder what you yell at the neighbor kids trying to retrieve their ball from your lawn.