Reach, the game, takes place from late July to the end of August, 2552. This comic begins before that, around May sometime, with Noble Team trying to stop the Insurrectionist/hostage situation (pages 1-3), then flashes forward to the end of August, when Reach is meeting its demise, with Jun escorting Halsey to Castle Base (which he was assigned to do near the end of the game). On page 13, Jun flashes back to May again when the rest of Noble Team was still alive. The time stamps should help if you read the comic in order.
It's kind of like an episode of Lost if you've ever seen one. Jun is in the 'present' for lack of a better word, trying to protect Halsey, while flashbacks reveal another story of Noble Team before it's demise. As we approach the conclusion of the comic, you'll start to see how these two stories are tied together.