Did they jump or were they pushed? Was Hamas's seizure of Fatah security offices in Gaza unprovoked, or a pre-emptive strike to forestall a coup by Fatah? After last week's turmoil, it becomes increasingly important to uncover its origins.

The fundamental cause is, of course, well known. Israel, aided by the US, was not prepared to accept Hamas's victory in last year's Palestinian elections. Backed by a supine EU, the two governments decided to boycott their new Palestinian counterparts politically and punish Palestinian voters by blocking economic aid. Their policies had a dramatic effect, turning Gaza even more starkly into an open prison and creating human misery on a massive scale. The aim was to turn voters against Hamas - a strategy of stupidity as well as cynicism, since outside pressure usually produces resistance rather than surrender.

The policy shocked even moderate western officials like James Wolfensohn, the former World Bank chief, whom the Americans had appointed to help Gaza's economy before the Hamas election victory. "The result was not to build more economic activity but to build more barriers," he said this week while explaining why he resigned in disagreement with US and Israeli strategy.

It is also well known that Hamas was as surprised by its election victory as everyone else and that it offered its rival, Fatah, a coalition government of national unity. The offer was refused. If this was done initially out of wounded pride, Fatah's rejection of Hamas's regularly repeated overtures increasingly appeared to be coordinated with Washington as part of the boycott strategy.

Reports have been circulating for months of a more sinister side to the boycott. According to them, the US decided last year on a plan to arm and train Mahmoud Abbas's presidential guard in a deliberate effort to confront and defeat Hamas militarily. Israel has already locked up several dozen Hamas legislators and mayors from the West Bank. The next stage was to do the same in Gaza but have Palestinians, rather than Israelis, run the crackdown.

Arming insurgents against elected governments has a long US pedigree and it is no accident that Elliott Abrams, the deputy national security adviser and apparent architect of the anti-Hamas subversion, was a key player in Ronald Reagan's supply of weapons to the Contras who fought Nicaragua's elected government in the 1980s.

Documents doing the rounds in the Middle East purport to have evidence for Abrams's "hard coup" strategy. One text recounts Washington's objectives as expressed in US officials' conversations with an Arab government. These are, among others, "to maintain President Abbas and Fatah as the centre of gravity on the Palestinian scene", "avoid wasting time in accommodating Hamas's ideological conditions", "undermine Hamas's political status through providing for Palestinian economic needs", and "strengthen the Palestinian president's authority to be able to call and conduct early elections by autumn 2007".

The document is dated March 2, less than a month after Saudi Arabia brokered the Mecca agreement under which Abbas finally agreed with Hamas on a unity government. The deal upset the Israelis and Washington because it left Hamas's prime minister Ismail Haniyeh in charge. The document suggests the US wanted to sabotage it. Certainly, according to Hamas officials whom a depressed Abbas later briefed, Abbas was told to scrap Mecca at every subsequent meeting he has had with Israeli prime minister Ehud Olmert or with US secretary of state Condoleezza Rice and Abrams.

Most ominously, the document of US objectives outlined a $1.27bn programme that would add seven special battalions, totalling 4,700 men, to the 15,000 Abbas already has in his presidential guard and other security forces, which were also to be given extra training and arms. "The desired outcome will be the transformation of Palestinian security forces and provide for the president of the Palestinian Authority to able to safeguard decisions such as dismissing the cabinet and forming an emergency cabinet," the document says.

Alastair Crooke, a former Middle East adviser to the EU foreign policy chief, Javier Solana, and current head of a research institute in Beirut, points out that Israel blocked some arms deliveries. It was wary of sending too many into Gaza for fear Fatah might lose them, as indeed has happened. In this sense, only part of the plan went ahead. (Britain has played a small part in helping Abbas's security forces. It has provided about £350,000 of "non-lethal" equipment this year for protecting the Karni freight crossing between Gaza and Israel.)

But Crooke says Hamas was irritated that the Mecca deal was being sabotaged, notably by the refusal of Mohammed Dahlan, Fatah's long-time Gaza strongman and head of the Preventive Security Forces, to accept the authority of the independent interior minister appointed to the unity government. "Dahlan refused to deal with him, and put his troops on the streets in defiance of the interior minister. Hamas felt they had little option but to take control of security away from forces which were in fact creating insecurity," Crooke says.

Ahmed Yousef, a Hamas spokesman, confirms the movement thought it had to move fast. In his words, last week's events were "precipitated by the American and Israeli policy of arming elements of the Fatah opposition who want to attack Hamas and force us from office".

While Hamas has successfully blocked the US-Fatah plans for Gaza, Abbas is trying to implement them in the West Bank by forming an emergency government. The policy is doomed since the constitution says such a government can only last 30 days. Parliament has to renew it by a two-thirds majority, and parliament is controlled by Hamas. The only sensible policy for Abbas must be to end the effort to marginalise Hamas. He should go back to the Mecca agreement and support a unity government. Even now, Hamas says it is willing to do so.

Where does all this leave the White House idea to involve Tony Blair as a Middle Eastern envoy? It creates a "coalition of the discredited" - Bush, Olmert and Blair - and sounds like something from a satire since Blair has no credibility with Hamas or most other Palestinians. Better to leave it to the Saudis to revive the Mecca deal, or wait until Abbas realises he has fallen into a trap. Neither common sense nor democratic principles, let alone time, are on Fatah's side.

j.steele@theguardian.com