When Iran was caught last September building a secret, underground nuclear enrichment plant at a military base near the city of Qum, the country’s leaders insisted they had no other choice. With its nuclear facilities under constant threat of attack, they said, only a fool would leave them out in the open.

So imagine the surprise of international inspectors almost two weeks ago when they watched as Iran moved nearly its entire stockpile of low-enriched nuclear fuel to an above-ground plant. It was as if, one official noted, a bulls-eye had been painted on it.

Why take such a huge risk?

That mystery is the subject of fervent debate in the White House and the C.I.A., and among European, Israeli and Arab officials trying to decode Iran’s intentions. The theories run from the bizarre to the mundane: Under one, Iran is actually taunting the Israelis to strike first. Under another, it is simply escalating the confrontation with the West to win further concessions in negotiations that have dragged on four months. The simplest explanation is that Iran has run short of suitable storage containers for radioactive fuel, so it had to move everything.

Whatever the cause, military officials say this is a tempting moment for the Israelis. The Obama administration clearly wants to make sure Israel does not take military action. In recent weeks it has sent the national security adviser and the chairman of the joint chiefs of staff to Israel to ensure there are no surprises like Israel’s 2007 strike on a nuclear reactor under construction in Syria. In that case, the Israelis gave the White House little warning of its decision to act.