CONFLICT ENTERS FOURTH DAY: Israel and Iran have begun a new round of attacks, as the conflict between the two heavily armed rivals enters its fourth day.

Former Vice President Mike Pence said on “State of the Union” Sunday that if Israel’s attack doesn’t somehow convince Iran to make major concessions in Trump’s diplomatic attempt to end its nuclear program, then the United States should be prepared to join the conflict.

Israel and Iran both have little incentive to stop and no obvious route to outright victory. Much depends on President Trump.

The president had opposed Israeli action against Iran but came to believe that Israel had reason to act and that the U.S. would have to lend some support.

Israel and Iran exchange airstrikes targeting military sites and nuclear facilities, with Trump comments on the conflict and expressing optimism for diplomatic resolution.

Iran launched a new wave of missile attacks on Israel early Monday, setting off air raid sirens nationwide as emergency services reported at least five dead and dozens wounded. Follow along for live updates.

"The full strength and might of the U.S. Armed Forces will come down on you at levels never seen before," Trump warned.

Israel’s attack on Iran was long in the making – the result of years of meticulous planning by Israel and days of high-stakes talks between Tel Aviv and Washington, officials told CNN.