The Israeli Air Force conducted a strike in Iran on Thursday, according to two sources with knowledge of the operation. It's not yet clear what Israel is targeting, but explosions were reported in Tehran.

Why it matters: Israel is directly attacking its biggest and best-armed adversary, without clear backing from the U.S.

President Trump publicly opposed an Israeli attack on Iran's nuclear sites on Thursday, saying he still believed a nuclear deal was possible.

It's not clear whether Israel has ordered strikes on Iran's nuclear program, or whether the U.S. opposed this specific operation.

But the Israeli strikes could be the start of a military conflict that poses grave danger to both Israel and Iran.

Driving the news: Sirens sounded across Israel on Thursday night. Israeli Defense Minister Israel Katz declared a special state of emergency across the entire country.

"Following the State of Israel's preemptive strike against Iran, a missile and drone attack against the state of Israel and its civilian population is expected in the immediate future," Katz said.

An Israel Defense Forces spokesperson said only "necessary" activities should take place in Israel starting Friday morning local time. That includes a ban on "educational activities, gatherings and workplaces, with the exception of essential businesses."

Behind the scenes: The U.S. notified several of its allies in private on Thursday that Israeli strikes were imminent and made clear it was not involved, one of the sources said.

The Trump administration had already told Israel it would not participate in any strikes on the nuclear program, Axios reported.

However, the U.S. has previously helped defend Israel from Iranian attacks and would likely do so again if this strike kicks off a retaliatory cycle.

The intrigue: Before the operation became public, U.S. Ambassador to Israel Mike Huckabee tweeted that he was at the embassy in Jerusalem and "will remain here all night," adding: "Pray for the peace of Jerusalem."

State of play: The extent and targets of the Israeli strike are not clear at this time.

Israel has been preparing for weeks for an operation to destroy Iran's nuclear facilities, but had previously told the U.S. it would wait to see what happens with Trump's nuclear talks.

A sixth round of U.S.-Iran talks had been scheduled for Sunday. It's unclear if negotiations will continue under the current circumstances.

The big picture: Iran has vowed to strike U.S. targets in the region in the event of any attack on its nuclear program.

The U.S. is in the process of withdrawing diplomats and military families in Iraq, Bahrain and Kuwait who could be in harm's way.

The U.S. has also boosted air defenses around several of its sources in the Gulf.

Even if Israel does not directly strike Iran's nuclear facilities, the U.S. could be caught up in Iran's retaliation.

Source: Axios.com