One key question is on President Trump’s mind these days, his advisers say: If the U.S. joins Israel’s war and drops its massive bunker busters, will they actually destroy Iran’s most heavily fortified nuclear facility?

Why it matters: Trump’s doubts Wednesday about the certainty of success are one reason he was still questioning whether to move forward with a strike.

The big picture: Trump wants to make sure such an attack is really needed, wouldn’t drag the U.S. into a prolonged war in the Middle East — and most of all, would actually achieve the objective of destroying Iran’s nuclear program, U.S. officials say.

• “We’re going to be ready to strike Iran. We’re not convinced yet that we’re necessary. And we want to be unnecessary, but I think the president’s just not convinced we are needed yet,” a U.S. official said.

Driving the news: If Trump enters the war it almost certainly would be to destroy the Fordow uranium enrichment site, which is built into a mountain south of Tehran.

• It’s at the top of Israel’s target list. But Israel lacks the 30,000-pound bunker-buster bombs needed to destroy it from the air, along with the B-2 bomber aircraft to carry them. The U.S has both within flying range of Iran.
• Israeli officials fear that if Fordow survives the war, Iran’s nuclear program will too.
• “We are the only ones who have the capability to do it, but that doesn’t mean I am going to do it,” Trump said Wednesday when asked whether he considers it essential to destroy Fordow. “I have been asked about it by everybody but I haven’t made a decision.”

Behind the scenes: Trump has specifically asked his military advisers whether the Massive Ordnance Penetrator (MOP) would destroy Fordow, a U.S. official said.

• Pentagon officials told Trump they’re confident it would, according to the official. But it’s not clear Trump was totally convinced.
• MOPs have never been used on the battlefield, though they went through several tests during development, current and former U.S. officials say.

Zoom in: “The bunker buster will work. It’s not a capabilities issue. We have the capability. But there’s an entire plan (for a possible attack). It’s not just drop a bunker buster and declare victory,” another top U.S. official said.

• “The end game for us is simple: no nuke,” that official said. “It may be different for the Israelis. And we would be willing to, if it’s within reason, go in there and do a surgical strike — maybe, if necessary” and if the president considers it “logical and effective,” the official said.

The other side: Israeli officials believe Trump will decide in favor of a U.S. strike, but contend they could cause significant damage to the Fordow facility even if forced to go it alone.

• Prime Minister Benjamin Netanyahu and Ambassador to Washington Yechiel Leiter hinted in recent interviews that the Israel Defense Forces have options beyond just airstrikes.
• One could be a risky commando raid. Israeli special forces conducted such an operation last September, albeit on a smaller scale, when they destroyed an underground missile factory in Syria by planting and detonating explosives.
• Now that Israel has full control of Iran’s air space and has dealt a heavy blow to Iran’s military, that option appears less extreme than it otherwise would.
• A U.S. official said the Israelis told the Trump administration that while they may not be able to reach deep enough into the mountain with bombs, they may “do it with humans.”

Between the lines: In recent days as Trump has been contemplating whether to join the war, his special envoy Steve Witkoff kept communicating directly with Iran’s foreign minister Abbas Araghchi, two U.S. officials and a source familiar with the issue said.

• “We continue to put it out there: ‘Let’s talk. Let’s talk, you know, because hope springs eternal. Some days Steve [Witkoff] says, ‘I got a call back. They’re interested in talking. And some other days, he says, ‘they didn’t call me back.’ So we’re not giving up on that one bit,” a U.S. official said.
• On Friday, the foreign ministers of France, Germany, and the U.K., along with the EU foreign policy chief, will meet in Geneva with Araghchi, a source with knowledge of the situation said.
• European official said EU foreign policy chief spoke by phone Wednesday with U.S. Secretary of State Marco Rubio and informed him of the planned meeting with his Iranian counterpart.

State of play: Trump held on Wednesday another Situation Room meeting with his top national security team on the war between Israel and Iran.

• “The president’s patience is running thin every minute that passes. The clock is really ticking for Iran, and all options are on the table,” a U.S. official said.
• Speaking with reporters ahead of the meeting, Trump said he hadn’t made up his mind about launching a strike against Iran and stressed he hadn’t closed the door on a diplomatic solution.
• Iran’s mission to the UN tweeted Wednesday that Tehran would “NOT negotiate under duress,” particularly with “a has-been warmonger clinging to relevance,” an apparent reference to Netanyahu. But there have been other indications the Iranians are willing to talk.

What to watch: U.S. officials said Trump believes maintaining ambiguity about his decisions applies more pressure on Iran.

• “I have ideas. But I like to take the decision at the last second,” the president said Wednesday.