The world starts at peace in 2000, with world tension at 0. It rises with every war goal a focus creates (+3, or +5 to annex or puppet a whole country), with Russia's Gathering of the Lands and with the attacks of 11 September (+15).
Gates: a focus that creates a war goal needs world tension above 10% (25% to annex or puppet a whole country), unless you are already at war with its target. A focus that invites a country into NATO or the Defence Union, or asks to join one, needs 15%.
The AI goes to war almost as soon as it holds a war goal, so it only receives one from a focus, an event or a decision once the world is very tense, or it is already at war with the target. A player gets it as soon as the focus allows.
How to play it: an early conquest needs someone else to raise the tension first - or a road that raises it itself.