President Trump came into office criticizing the U.S. wars in Afghanistan and Iraq and promised he would try to avoid foreign military engagements. Yet this month the White House has been talking as if conflict with Iran is suddenly on the table. Trump tweeted over the weekend, "If Iran wants to fight, that will be the official end of Iran."

But it's not clear if U.S. officials have evidence that Iran "wants to fight" or why the Pentagon has dispatched additional ships and bombers to the Middle East.

National security adviser John Bolton and Secretary of State Mike Pompeo have been especially aggressive in their Iran rhetoric.

Pompeo and other top officials briefed members of the House and Senate in private on Tuesday. Acting Defense Secretary Patrick Shanahan told reporters that the U.S. warnings and deployments have already made Iran think twice about starting anything. "This is about deterrence, not about war," Shanahan said .

But Iranian Foreign Minister Mohammad Javad Zarif told CNN the U.S. is "playing a very, very dangerous game" by placing more military assets in the region.

Here's a look at how the talk has turned so bellicose and the risks involved.

What changed between the U.S. and Iran?

The first Sunday night in May, the White House issued a statement from Bolton, saying the U.S. has seen "a number of troubling and escalatory indications and warnings" tied to the Iranian regime threatening U.S. and allied interests. Bolton said the U.S. was sending an aircraft carrier strike group and land-based bombers to the Persian Gulf in response.

Later, administration sources said intelligence photos showed Iran had loaded missiles onto small boats run by Iran's Islamic Revolutionary Guard Corps, which the U.S. last month declared a terrorist organization. But then the Iranian forces reportedly unloaded some . U.S. allies in the Gulf last week said oil tankers suffered damage from sabotage attributed to Iran. Iran has denied plans to attack U.S. interests but said it is prepared to defend itself against the United States.

Iran and the U.S. have been adversaries for decades. Why is this happening now?

The Trump administration is trying to pressure and isolate Iran.

A year ago, Trump fulfilled a campaign promise by pulling out of the Obama-era agreement to limit Iran's nuclear program. That was a deal between Iran and world powers to ease economic sanctions in return for Iran putting limits on its nuclear program. Trump says it's not tough enough. Since pulling out, the U.S. has been pushing hard for countries to stop doing business with Iran in the hopes it will change its behavior in the region, like stop supporting militias in Lebanon, Yemen and elsewhere and stop developing missiles. Pompeo says Iran needs to act "like a normal country."

One of the U.S. officials raising alarms about Iran threats is Bolton, who joined the administration as national security adviser a year ago and has in the past supported forcing a regime change in Iran. But administration officials insist the goal now is only to change the regime's behavior. That would include, officials say, renegotiating the nuclear deal. Iran rejects the idea of reopening the deal.

Why are other countries resisting the U.S. push?

First, the Trump administration has split from other world powers on the nuclear deal. The U.S. is the only signatory to pull out, while others — Britain, France, Germany, Russia, China, the European Union — say the agreement is working to keep Iran from getting close to building nuclear weapons. They're working to keep it going, even though companies in those countries could get cut off from business with the U.S. if they do business with Iran. China, Iran's biggest oil buyer, has tried to continue importing Iranian crude.

As the U.S. increases pressure and warns of military action, European leaders seem to be getting more worried. They fear Iran will just decide that since it's not getting the sanctions relief and economic benefits promised under the nuclear deal, it will just resume its previous nuclear activities . Iran has threatened it could do so next month if Europe doesn't save the deal.

Europeans remember misleading U.S. warnings about the threat of Iraqi weapons before the U.S. invasion of Iraq in 2003.

How real is the risk of war?

Both Iran and the United States are on the record denying they want a war and it's hard to see how it would be popular in either country. But the military situation in the Middle East is inherently unstable, and there are multiple potential flashpoints. U.S. and Iranian military forces are in close proximity across the region.

There's a risk of miscalculation or accident. U.S. and Iranian boats crisscross in the Persian Gulf, where Iran captured some U.S. sailors in 2016 . Iranian officials have threatened to block the narrow Strait of Hormuz . Pro-Iranian militias move in the same cities in Iraq as U.S. troops and have attacked them in the past. They're also close to each other in Syria.

The U.S. has major military bases in Qatar and Bahrain, just across the Gulf from Iran.

Some of Iran's hard-liners might welcome a conflict with the U.S. because it would undermine moderate Iranians who want to engage with the West.

And a small conflict could grow. Iran could engage in underground attacks around the region or outside it. Israel or Saudi Arabia might take the opportunity to attack their big regional rival.

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