Their unity pact signed yesterday is being called surprising, historic and controversial, but neither Fatah nor Hamas, the rival Palestinian faction with which it is making amends, have offered much in terms of details.

Analysts have described the Egyptian-brokered agreement, which will be celebrated in a ceremony today in Egypt's Nasser city, as a tactically beneficial compromise for both in light of popular calls for reconciliation, regional political upheaval and attempts at earning international recognition for a Palestinian state.

But in the case of Hamas, they say, pressure to sign up was especially pronounced.

"It's because the Palestinian Authority, Fatah, has won big in its competition with Hamas," said Yaron Ezrahi, a professor of political science the Hebrew University of Jerusalem.

The Islamist group, he said, feared being seen as failing to back its Fatah rivals' attempt at gaining UN recognition for a Palestinian state.

Fatah's "expected achievement in September is so historical that Hamas doesn't want to be left behind, because it will endanger its position in the Palestinian community," Mr Ezrahi said. "Hamas will be marginalised if does not join in during the moment that a Palestinian state is recognised."

He and others point to Hamas and its growing concern of being labeled a spoiler, such as its perceived snubbing in March of Mahmoud Abbas, the Palestinian Authority (PA) president, and the Fatah faction that he chairs.

Mr Abbas accepted an invitation to visit the Hamas-controlled Gaza Strip after mass Palestinian demonstrations called for unity. Soon after, a flurry of mortars fired from Gaza struck Israel - a move widely viewed as a deliberate attempt by Hamas to thwart the Palestinian leader's visit.
But on March 18, mass protests escalated in Syria, destabilising Hamas' home in exile and the rule of the group's once reliable patron, Syrian President Bashar al Assad.

This was a critical point that prompted Hamas to favour a unity accord, said Majid Shihade, a professor at Birzeit University's Ibrahim Abu-Lughod Institute for International Studies.

The London-based Arabic daily Al Hayat reported on Saturday that Khaled Meshaal, Hamas' leader, and his Damascus-based politburo were planning to move to Qatar, and his deputy, Musa Abu Marzouk, to Egypt. Hamas has denied the report.

But, Mr Shihade said: "If Syria is overtaken by revolution, Hamas will have to run to a very weak place for cover, like Sudan. And that would have meant Hamas operating from a weaker position, so they took advantage of the situation by going through with the unity deal."

Moreover, he noted, Mr al Assad appears to have supported Hamas' entry into the unity pact as a gesture to gain support for his faltering rule from Egypt and other Arab countries. Syria's relations with these countries have long been strained, largely because of its ties with Iran.

Egypt's transitional government has taken on a foreign policy independent of Washington and Israel, alarming both with diplomatic gestures to their foremost regional antagonist, Tehran.

Another example of this, analysts say, was that Egypt offered Hamas incentives to agree to the unity pact that went beyond anything ever offered by the country's former president, Hosni Mubarak, a staunch US ally and dependable friend of Israel's.

Mkhaimar Abusada, a professor of political science at Gaza's Al Azhar University, said Hamas received from Egyptian mediators promises to effectively break Israel's blockade.

Then, a day after the accord was struck, Egypt's foreign minister, Nabil al Arabi, announced that the country's Rafah border crossing with Gaza would be permanently opened. Israel, with the help of Mr Mubarak, blockaded Gaza after Hamas violently took control of the area from Fatah forces in 2007.

"If they agreed, the siege would be ended and Rafah would be opened, in addition to a number of economic incentives to sign agreement," which Mr Abusada said included aid for rebuilding the destruction wrought by Israel's three-week war on Gaza two years ago.

This also could help Hamas improve relations with Egypt, Saudi Arabia and possibly Europe, he added. "Even though Hamas has built its empire in the Gaza Strip," he said, "it has gone nowhere with the international community and Arab counties."

Still, there appear to be risky quid-pro-quos involved in Hamas' acquiescence to the pact.

While the group is thought to have been granted exclusive security control over Gaza at least until the Palestinian elections expected in the coming year, it also appears to have agreed to Fatah's security coordination with Israel during that time. This has been used to arrest Hamas members and is designed to thwart Hamas' stated policy of fighting Israel, which the Islamist group has refused to recognise.

This could make Hamas look hypocritical in the eyes of Palestinians and would likely hurt it during the upcoming elections, said Khader H Khader, a political analyst who works at the Jerusalem Media & Communications Centre.

"Hamas will lose big time with this," he said.

"Why? Because they will, in effect, become less and less distinguishable from Fatah in their policies. If you tell me I want to resist and yet you're not doing anything, then why should I believe you? Why put the people of Gaza under siege for five years if you accept the Egyptian agreement?"