Five Long Years
From Halopedia, the Halo wiki
Several Marines run through a battlefield and kill a group of oncoming Elites. A Marine is killed when a nearby wall explodes and he is engulfed by debris and plasma. The surviving Marines recover quickly and start to move forward.
They confront a lance of slowly advancing Elites on the other side of a wall of debris. A few of the Marines yell out as they arrive. One Marine quickly joins in and starts to fire upon the Elites, only to be shot by a plasma bolt to the chest.
Three Marines fire their weapons. The camera then cuts to a shot of several Pelican dropships arriving with several Warthogs racing through the drop zone. Marines then charge out of one of the landed Pelicans. There is a cut to a Marine hitting an Elite from behind, so that he falls down and is shot dead by another Marine. Then it cuts to a panorama, showing nine Marines cheering and a Warthog overlooking the battlefield; a destroyed city with a burning CPV-class heavy destroyer in the distant sky, presumably shot down by UNSC forces.
Fade to black
An Elite pierces a Marine with his energy sword. UNSC troops are overwhelmed by plasma fired by Elites and Banshees. The camera then cuts to a scene where three Elites jump over a crumbled wall, the two in the background wielding plasma rifles, the one in the foreground activating two energy swords.
Two Covenant destroyers begin to glass Harvest.
A surviving Marine attempts to crawl to safety when an Elite approaches and stabs him with his energy sword.
Camera turns toward the skies and another ship appears and activates its energy weapon. White follows.