WASHINGTON – President Donald Trump has threatened Iran for years, and often says he is willing to use force – but allies and aides say it would take an exceptional provocation for him to act, given his equally long criticism of "endless wars" in Iraq and Afghanistan.

"We are ready for the absolute worst and we are ready for sense, too," Trump said Monday, adding that he is also open to the idea of new talks with the Iranians over their nuclear program.

"I am just going to sit back and wait," Trump said.

In the meantime, Trump aides said Iran's latest actions – the seizure of a British oil tanker, the arrest of alleged pro-American spies – are unlikely to provoke a military response.

Yet there are circumstances under which Trump could decide to use force, said current and former aides.

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More aggressive Iranian actions – an attack that kills Americans, a mass casualty attack on an ally, a closure of the Strait of Hormuz to international shipping traffic would lead to an equally aggressive response by Trump.

"His position is he doesn't want to get us into new wars," said Fred Fleitz, former chief of staff to Trump National Security Adviser John Bolton. "But he's prepared to use force if necessary."

Throughout his presidential campaign, Trump frequently hit predecessor George W. Bush over costly military action in Afghanistan and Iraq. During a foreign policy speech in September of 2016, Trump talked about "how to avoid the endless wars we are caught in now."

Trump also hammered Iran during that campaign, and during his tenure in the White House. He has attacked predecessor Barack Obama for signing an agreement in which the U.S. and allies reduced economic sanctions on Iran as it gave up the means to make nuclear weapons.

When Iran detained a U.S. ship in January of 2016 – and released pictures of American sailors on their knees, hands behind their head – Trump tweeted that it "humiliated the United States with the capture of our 10 sailors. Horrible pictures & images. We are weak. I will NOT forget!"

Trump's decision to withdraw the United States from the Iran nuclear agreement – and impose new sanctions on Tehran – triggered many of the new tensions. In recent weeks, Trump has held out the possibility of talks on a new nuclear agreement with new terms.

These kinds of competing impulses – threats on Iran, caution over military action – played out last month when Iran shot down an unmanned U.S. drone.

At one point, Trump authorized military action – then changed his mind, saying a strike against Iranian radar installations would probably cause deaths, and would not be proportional to shooting down a drone.

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Trump has authorized two missile strikes in Syria over its use of chemical weapons, but those were limited strikes that targeted aircraft facilities.

Amid renewed tensions with Iran, U.S. military assets have stepped up patrols in the region of the Strait of Hormuz, but are not acting as if military action is imminent.

U.S. Navy ships, patrol planes and drones have been keeping watch over critical shipping lanes in the Middle East and providing information on Iranian activities to captains of American and allied vessels.

U.S. Central Command, which oversees American forces from the Middle East to Afghanistan, has dubbed the initiative “Operation Sentinel.” It includes warships operating in the Persian Gulf and those on station in the Gulf of Oman that provide information to commercial ships on the action of Iranian military and paramilitary forces.

“We have patrol aircraft operating in international airspace monitoring the situation within the Strait of Hormuz,” said Army Lt. Col. Earl Brown, a spokesman. “U.S. Naval Forces Central command has been in contact with U.S. ships in the area to ensure their safety.”

British officials, while demanding Iran release its tanker, are stressing a diplomatic solution, and downplaying the use of force.

The British government said Monday that UK officials have been in touch with the United States, France, Germany, Italy, Finland, Spain and Denmark in recent days. British Foreign Secretary Jeremy Hunt said they are planning to put together a “European-led maritime protection mission to support safe passage of both crew and cargo in this vital region.”

“The new force will be focused on free navigation, bearing in mind that one fifth of the world’s oil, a quarter of its liquefied natural gas – and trade worth half a trillion dollars –passes through the Strait of Hormuz every year,” Hunt told members of Parliament.

He added that the new effort would not be part of the U.S. "maximum pressure policy," which relies on increased economic sanctions the Trump administration imposed after pulling out of the Iranian nuclear agreement.

The United Kingdom and other European nations still support and want to preserve the nuclear agreement.

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The Pentagon has said its “Operation Sentinel” would involve a multi-national build-up of navy forces, with partner nations escorting their own ships through the straight with coordination and surveillance help from the United States.

John Rood, under secretary of defense for policy, said on Saturday that he believed other nations were willing to participate, though reports had suggested some countries were reluctant to commit more military assets to the region for fear it would inflame tensions with Iran further.

The United States already has sent a carrier battle group, B-52 airplanes and air and missile defense systems to the area. Last month, the administration announced it was deploying 1,000 to the region. On Friday, the Pentagon said some of those are heading to Saudi Arabia. Defense officials declined to say how many.

“It would be a grave mistake on their part to miscalculate,” Rood said of Iran, according to a defense report.

As for the United States, former White House Press Secretary Sean Spicer said Trump will be cautious about military action for a simple reason: His criticism of Iraq and Afghanistan. "He's going to be very careful about doing something that would create the very circumstances he campaigned against," Spicer said.

For Trump, it is still a wait-and-see situation.

"It's getting harder for me to want to make a deal with Iran because they behave very badly," he said Monday. "They are saying bad things and I will tell you it could go either way."