The case for a U.S. military strike against Iran’s nuclear facilities remains weak.

Washington does not know where all the underground uranium enrichment facilities are. They are tough to hit. Even if destroyed, they can be rebuilt.

A strike might actually set back Iran’s bomb program, but just one to three years. If President Barack Obama authorized one, especially without United Nations sanction, he could be viewed as repeating President George W. Bush’s pre-emption mistakes, increase risks of an Iranian response and possibly help President Mahmoud Ahmadinejad bolster his domestic political position.

In fact, we don’t even favor keeping force on the table. You don’t threaten a war you know is a bad idea. Doing so puts pressure on Obama to strike if and when diplomatic and economic approaches fail to rein in Iran’s program.

In addition, even if the charge is unfair, Obama’s critics at home could accuse him of weakness, increasing the pressure on him to consider an attack. America’s credibility might also be weakened, since Washington would be seen as having its bluff called. Future scenarios in which the United States might wish to credibly threaten an attack against an adversary, perhaps even Iran, would then be harder to handle.

So the Obama administration would do well to stop talking publicly about a military option and, instead, tell regional allies a strike is highly unlikely unless Iran’s behavior becomes far more egregious.

But what about Israel?

How should Washington think about the possibility that the Jewish state, having long heard Ahmadinejad’s belligerent threats, might itself strike Iran? This could implicate Washington in the eyes of many, for Israel probably would not act without giving the United States tactical warning — if only to protect its own planes.

Yet, as long as Washington keeps the military option on the table, it is implicitly legitimizing Israel’s right to do so, too.

Just as it makes little sense for the United States to attack Iran’s nuclear facilities, it is also not a good idea for Israel — now or in the foreseeable future, under any plausible scenario.

The same military arguments are relevant. In fact, with its smaller air force and greater distances to fly, Israel would surely be less effective in demolishing Iran’s underground facilities than America would — meaning that Iran’s efforts might be delayed only a year or two. Tehran would likely lash out at the United States as much as at Israel — especially with 200,000 American GIs deployed in the region.

Also, Obama’s efforts to improve relations with the broader Muslim world, demonstrated last year with his speech in Cairo, would be greatly complicated by what many would see as yet another American attack on a Muslim state.

A few Gulf states might privately welcome the strike. But most of the Islamic world would ask why a state that the Federation of American Scientists says has 80 nuclear weapons of its own was justified in attacking another that has yet to produce one such weapon.

This double-standard argument could resonate from Morocco to Indonesia. America’s most vital partners in the war against Al Qaeda would have a harder time working closely with Washington on counterterrorism and bolstering defensive preparations that Iran’s belligerence has helped produce in recent years.

It would also undermine governments in key regional states that welcome our presence and help. Jordan, Turkey, Pakistan and Indonesia would again have to explain to their angry populations why security partnerships were necessary with a foreign power too friendly to Israel and too inclined to strike any Muslim state that gets out of line.

This battle of perceptions, now starting to work in our favor, could turn against us.

An Iranian nuclear bomb would clearly be a bad thing. We must continue to do everything in our power, on the diplomatic and economic-sanctions fronts, to make it less likely — and to deter it if we have to.

To discourage Israel from any attack, we need to remind its leaders of three key points:

• U.S. security is implicated in how this is handled, too, because our troops and interests are exposed and subject to Iranian reprisal after any strike.

• An Iranian bomb would strengthen, not weaken, the U.S. commitment to Middle East security. Washington would need to more forcefully project a U.S. nuclear umbrella policy to protect regional friends. Iran would be put on notice that it should assume any nuclear strike against other countries in the region would lead to U.S. retaliation.

• Israel would actually be less alone if Iran got the bomb, since it would not be the only state threatened.

Finally, should Iran move to build nuclear reactors and plutonium-reprocessing facilities capable of producing enough plutonium for a large number of bombs, a different conversation could occur about whether military action made sense. Aboveground facilities, like reactors, are easier to strike — especially before they become operational.

These arguments alone cannot prevent Israel from carrying out an attack. It must make its own decisions.

But it would be more difficult for Israel to strike if Washington made its opposition to a strike clear and strongly recommitted to the long-term security of its friends in the region.

The Obama administration should have this conversation with Israel quietly — before we wake up one morning to a fait accompli.