An action that can be interpreted as an incitement to war. For example, crossing a hostile border with a force of armed troops would clearly be an act of war. An outright attack upon a ship of war is also widely recognized as an act of war.

The line between a terrorist act and an act of war can sometimes be fuzzy. Terrorism is generally directed at civilians. Also, there is a serious question of whether an act of war can be committed by any entity except a legitimate government.

The question of what constitutes a legitimate government is left for another node on another day.

artfuldodger: Unfortunately, E2's search function apparently blows goats, especially when it comes to compound words. (This makes it particular unsuitable for the German language, but that's neither here nor there.) I searched for "artful dodger" and you didn't come up. I searched for "artful" and you didn't come up. I searched for "dodger" and you didn't come up. I assumed that a sane search function would have turned up the collision, but I was wrong. Rest assured, I didn't do it just to confuse people ;-)!

So, to abuse the analogy even further, is it an act of war to move into inhabited territory when the inhabitants are, for all intents and purposes, invisible?