Like a mismatched couple who live together miserably before breaking up violently, Fatah and Hamas were doomed partners from the start.

But what triggered the final, violent rupture between the Palestinian factions?

Last week's virtual civil war, in which about 100 Palestinians were killed in the Gaza Strip, was a crisis foretold.

Since the founding of Hamas in 1987, there has been a deep fault line between its Islamic fundamentalist creed and the more secular side of Palestinian life represented by Fatah - between Hamas followers perceived as disciplined and Fatah bureaucrats perceived as inept at best.

The January 2006 landslide electoral victory of Hamas and its entry into the governing Palestinian Authority exacerbated the differences. Hamas sought to implement its mandate. Fatah refused to share power after ruling Palestinian affairs for 40 years.

A unity agreement brokered at Mecca in Saudi Arabia in March was supposed to tame the hostilities. Some experts said the agreement never had a chance because the parties already were deep into fighting mode...

Ghaith al-Omari, a former adviser to Palestinian president Mahmoud Abbas, now a fellow at the New America Foundation think tank in Washington, spoke to his contacts in the Middle East last week and heard two versions about why Gaza boiled over.

One theory is that the Hamas leaders who lost out in the Mecca agreement, principally Foreign Minister Mahmoud Zahar and former Interior Minister Said Sayyam, green-lighted the violence but without the approval of Hamas higher-ups, including prime minister Ismail Haniyeh and Khaled Mashall, the leader-in-exile in Damascus, Syria.

"One thing Mecca didn't do was deal with the security sector. Who would be in charge?" Omari said. "So Hamas decided to just take over the whole security sector. Politically, I think, they bit off more than they can chew. They are stuck now with Gaza with no resources, and more than likely Israel will intensify its actions."

Another theory is that Hamas struck now - preemptively - because it feared Fatah was building up militarily using U.S.-supplied small arms and ammunition deliveries facilitated through Israel, both of which occurred in recent months.

"That is Hamas' line," Omari said, "but I'm not sure I buy it. I think this started out as a limited engagement, gained momentum, and just snowballed..."

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