The line comes from Halo's capture-the-flag multiplayer, where an announcer voice feeds players objective callouts during matches. Clips of the game's iconic announcements circulate widely among shooter fans, who reuse them in montages, streams and memes much like other objective callouts like bomb has been planted and tactical alerts such as enemy spotted. Because the phrase is short and commanding, it works as a generic rallying cry for defense, which is why it still appears in gaming edits years after its debut.