Washington — By putting Iran “on notice” for its aggressive behavior, President Trump has taken aim at a country that’s opposed by many U.S. allies. But he has begun this confrontation without much preparation or strategic planning, continuing the haphazard pattern of his first two weeks in office.

Iran is a convenient enemy for Trump. Israel and the Gulf Arab states share the administration’s antipathy toward Iran, and the regime’s hard-liners gave Trump a pretext with a ballistic-missile test last weekend that arguably violated a U.N. Security Council resolution.

Trump’s challenge also comes at a moment when Russia, Iran’s only major ally, is seeking better relations with the new administration. That may be a useful point of leverage. Some American, Israeli and Arab officials hope Russia might be persuaded to accept limits on Iranian behavior as the price of rapprochement with the U.S. But some senior intelligence officials are skeptical.

Confronting Iran carries significant dangers. The U.S. Central Command has thousands of troops in Iraq and the Gulf that could be vulnerable to Iranian reprisals. The White House, however, didn’t coordinate its actions with Centcom before national security adviser Michael Flynn announced Wednesday his nonspecific but menacing “notice” about Iran’s “destabilizing” behavior.

In a tweet Thursday, Trump echoed Flynn’s comment that Iran should be grateful for the 2015 nuclear agreement negotiated by President Obama, which Trump termed a “terrible deal,” rather than continuing its aggressive actions. The administration appears to be considering new sanctions, but since taking office, Trump hasn’t moved to revoke the deal itself.

Iranian officials launched rhetorical counter-volleys. A Foreign Ministry spokesman described Flynn’s warning as “repetitive, baseless and provocative.” But the Iranians, too, avoided any suggestion that the nuclear agreement was at risk.

Trump’s goal of curbing aggressive Iranian behavior in the region has wide support, including among many countries that backed the nuclear deal. Arab nations argue that Iran has destabilized regimes across the Middle East, and that its proxies now control Baghdad, Damascus, Beirut and Saana. Flynn’s statement cited an attack last week by Iranian-backed Houthi rebels on a Saudi vessel off the Yemen coast.

“I don’t think we are so much looking for a fight as responding to lethal provocations,” argued one senior U.S. military official. He noted that in addition to attacking the Saudi ship, the Houthi rebels have been mining waters near the strategic Bab el-Mandeb Strait.

But U.S. and foreign officials caution that any attempt to contain Iran needs to be carefully planned and implemented. Iran is a hardened adversary, despite its political isolation. Any confrontation has to take into account Iran’s strong position in Syria and Iraq, and its ability to thwart Trump’s pledge to eradicate the Islamic State there.

“They wanted to send a message, but they have no idea what it means,” says a top Republican former foreign policy official.

With just two weeks in office, the new administration hasn’t had time to fill some key national-security posts, let alone plan a strategy. Take Syria: Administration officials don’t like Obama’s strategy, but they don’t yet have an alternative.

The Trump team has explored partnering with Russia, and even considered contacts with the regime of President Bashar Assad. Some Syrian opposition officials have urged the U.S. to work with Russia (and, implicitly, the Assad regime) in a joint partnership against the Islamic State. One opposition leader told me this week there’s hope that Moscow will curtail the operations of the roughly 5,000 Iran-backed Syrian Shiite militiamen there.

But Iran holds some choke points. Its strongest leverage is in Iraq. With the victory over the Islamic State in Mosul probably six months away, the Iranians can mobilize thousands of Iraqi Shiite militiamen across Iraq. U.S. advisers are vulnerable to attack by these Iran-backed militias, as happened a decade ago in Iraq.

The complex order of battle in Syria was described Thursday by Ahmed al-Jarba, who leads an opposition group called the Syrian Elite Forces. He said in an interview that his roughly 3,000-man Sunni Arab group is now being trained inside Syria by U.S. Special Operations Forces, alongside Syrian Kurdish fighters, in preparation for the coming assault on Raqqah. He said his group also had “good and balanced relations” with Russia, even though it opposes Assad and Iran, Russia’s partners. That’s a tangled web.

Moderating the Iranian threat in the Middle East has been an American aim since the 1979 revolution. Arabs and Israelis alike will cheer Trump’s hard line. But Iran is among the toughest foreign policy challenges Trump will face, and he should be careful to avoid ill-planned early actions that would make it his Bay of Pigs.

— David Ignatius is a columnist for Washington Post Writers Group.