World at War

At the beginning of your upkeep, all delayed triggered abilities created by rebound effects trigger. You may handle them in any order. If you want to cast a card this way, you do so as part of the resolution of its delayed triggered ability. Timing restrictions based on the card’s type (if it’s a sorcery) are ignored. Other restrictions are not (such as the one from Rule of Law).

Multiple World at War effects are cumulative, as long as they’re cast early enough. The first one modifies the turn structure to this: beginning phase, precombat main phase, combat phase, postcombat main phase, [new combat phase], [new postcombat main phase], ending phase. Each subsequent one inserts another combat phase and main phase into the turn after the original postcombat main phase and before the newest combat phase.

Unlike other similar cards (such as Relentless Assault), World at War doesn’t untap the creatures that have attacked this turn when it resolves. Rather, those creatures untap when the new combat phase created by the spell begins. All creatures that attacked this turn untap, regardless of which combat phase they attacked in (if there have been more than one) or who controls them (if you’re playing a Two-Headed Giant game, for example).