(This describes medals in the Halo franchise rather than the Daniel X first strike attack which killed most of the population including Willy Joe Emma and Dana)
The first strike. The First Strike medal is a medal in Halo: Reach and Halo 4 obtained by getting the first kill in a multiplayer match.
The medal is a reference to Halo: First Strike and the common term "First Blood" in other games.
A bug in the multiplayer beta causes this medal to reset upon leaving a host migration; the first player to score a kill thereafter earns the medal.
The achievement "Yes, Sensei" is unlocked upon earning this medal in matchmaking. Once the achievement is gained, you unlock armor from Halo Waypoint.
Although the description of the medal is getting the first kill of a game, it will actually be received for getting the first kill of a round. It is therefore possible to get the medal multiple times in games with more than one round.