This paper analyzes the deeper reasons and the political motives which made the Hamas leadership in Gaza opt for a military strike against the Fateh-dominated security forces as well as against Fateh’s leadership and some of its militants and members in June 2007. At the same time, a critical look is taken at some of the violent action taken during Israel’s war against Gaza (2008/2009) by Hamas against Fateh-members accused of being Israeli spies or collaborators. This paper starts with the assumption that Hamas had been able to take over full control over Gaza already in 2005, after Israel’s withdrawal from the Gaza Strip. Hamas, however, rather opted for political participation and ran in both local and parliamentary elections in the period between December 2004 and January 2006. The successful showing of Hamas in the parliamentary elections transformed it from an opposition movement into the ruling party. This very transformation and the concomitant refusal of Fateh to accept its defeat at the ballot boxes created the basis for the subsequent fighting between the two movements. While Hamas tried to keep up its principled stand of not attacking other Palestinians until early 2007, despite continuous provocations on the part of Fateh, the situation changed after the breakdown of the unity government of spring 2007. When it became obvious that Fateh was preparing a coup against the elected Hamas-government in Gaza, a coup which was supported if not completely organized by international (USA) and regional actors (Egypt, Israel), Hamas felt cornered and forced to act. The previous challenges against Hamas by Fateh in both the West Bank and Gaza, on the level of strikes, a full media war, the creation of chaos, and even assassinations, contributed to this. Still, there are voices inside Hamas, which criticize the resort to force in dealing with the serious challenges posed by Fateh. This paper will present and critically analyze both positions, the one justifying the fighting against Fateh as a last resort, and the one criticizing it as being the wrong reaction which made it give up a central ideological and political principle and ultimately backfired against Hamas. The paper will use internal Hamas papers, published analyses and interpretations, as well as interviews with leaders and activists in order to arrive at a satisfactory explanation at what happened in 2007 and in early 2009.