Question from a friend: Why are Hamas and Fatah fighting? How will it affect Israel?
Short answer - because they're savages.
Longer answer - Palestinian society seems to have two groups of elites, one the fanatic wing of the Sunni clergy and the other everybody else. The umbrella group for everybody else was the PLO. When the PLO became institutionalized as the PA, the Palestinian Authority, under the Oslo Accords it was assumed that it would be essentially a mere renaming since it never occurred to anyone back in 1993 that Arafat and the PLO would ever not be re-elected.
When Hamas won a landslide electoral victory two years ago, the PA wound up with an insane constitutional arrangement - a president of one party and a legislature of another party. In the US the president is also Commander in Chief of the military - there too. Here the military is at least nominally kept out of politics. There the military arises as political militias representing political factions. There the "congressional" party, Hamas, also has a strong militia. Here the Democrats and Republicans snipe at each other using talk shows and speeches. There the parties snipe at each other from rooftops using AK-47's. Did I mention that they're savages?
The polite politically correct answer to how it affects Israel is that since the Abbas and Fatah side are willing to negotiate with Israel and the Hamas side are not, that a defeat for Fatah would be bad for Israel. With Hamas there would be no partner with whom to negotiate peace. The assumption is that Abbas, Arafat's handpicked successor, and Fatah are interested in peace and willing to negotiate for it in good faith. For anyone not suffering from amnesia, it is clear that the Palestinians have never negotiated in good faith and have no interest in peace. The Oslo Accords were followed not by peace but by the second intifada.
For Israel Hamas is much the better prospect because they are unwilling to lie about their intentions. The Europeans, because of their own Muslim populations, are unwilling to fund Islamists even against Israel. The European Union was the main source of support of the Fatah-led PA. The aid cutoff has crippled Hamas and led to some of the current instability. The PA funded and politically supported by Europe and possessed of a powerful public relations machine was a formidable opponent. The Hamas PA without funds (not from the other Arabs either) and screaming Muslim jihad slogans is impotent - except internally. The Fatah PA has been so consistently corrupt and a failure that the Palestinians turned to Hamas because it was an outsider and seen as not corrupt.
The effect on Israel that I am concerned about is not what will happen during a Hamas - Fatah civil war but afterwards. The best outcome would be an endless bloody draw. But after such a war, both sides would have large numbers of combat-experienced troops. Veterans of combat are almost invariably better troops than those who have not been in combat. One would have to hope that the war's end left both sides intact enough to be more concerned about one another than about Israel.