During the Middle Ages, intellectuals endlessly debated the question: How many angels can dance on the head of a pin? To this we add a contemporary version: How often will Israel kick Washington in the teeth before it says “Enough!”

Our question, of course, is occasioned by the visit of the US vice president, Joe Biden, to Jerusalem and Ramallah this week. Mr Biden was dispatched by the White House to assure Israelis of America’s commitment to their security.

Rather than offer its gratitude, Israel announced that it would build 1,600 homes for Jewish settlers in East Jerusalem while its prime minister, Benjamin Netanyahu, was meeting with Mr Biden. It was a resounding slap in the face to an administration trying to take a modest step in the peace process. Yet following precedent, ever-polite Washington replied to Israel’s provocation with tepid disapproval.

Saeb Erekat, the chief negotiator for the Palestinian Authority, offered a more honest assessment of the damage Israel had caused. After Arab heads of state so recently endorsed proximity talks led by the US special envoy George Mitchell, “this is destroying our efforts”, Mr Erekat said. “It’s really a disastrous situation. I hope this will be an eye-opener to the international community.”

The illegal settlement of Palestinian territories and the inability to achieve a two-state solution stirs resentments far beyond the occupied territories or even the wider region. The conflict must have an international solution and the weight of an honest broker to force both parties to compromise. But how can the Palestinians trust the US as an honest broker when it continues to provide tacit approval to Israel’s intransigence and allows itself to be humiliated in the process?

There is, however, a logic behind why the US lets its international prestige be so damaged by these Israeli provocations. The most immediate answer, of course, is electoral politics. Congressional elections will take place in November, and Mr Obama is already positioning himself to run for president again in 2012. His party’s coffers rely heavily on Jewish donors, who must not be alienated by an open spat with Israel.

We regularly hear other explanations for Washington’s masochism. One is the control exerted by the Israel lobby. Another is that Americans and their government have more empathy with democracies. All these explanations for American spinelessness are, to one extent or another, misleading of downright wrong. Where they really run aground, however, is in their implication that the US political and diplomatic game is rigid and unchanging. Certainly, the game is tainted by corruption and craven self-interest, but it is not fixed. It can be influenced by those with enough know-how, persistence, and yes, money.

Washington will continue to take Israel’s humiliations lying down until those in the American government with the vision of a more just Middle East receive the political cover they need to fight back, not only from like-minded people in America but from their supporters abroad. They’re waiting.