President Donald Trump is veering from his campaign promises to destroy ISIS at all cost, says former presidential candidate Ron Paul.

On Saturday, a U.S. Navy fighter jet shot down a Syrian war plane after it attacked rebel forces being supported by Washington.

The F-18 launched from the USS George H.W. Bush in the Mediterranean Sea near Israel and shot down a Syrian SU-22 fighter over Syrian territory. This is the first time the U.S. has engaged in an air-to-air shoot-down since the U.S. attack on Yugoslavia in 1999.

In response to the U.S. attack, the Russians have suspended the use of the military hotline and announced that they will view all unidentified aircraft operating in the vicinity of Russian military aircraft as potential targets.

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Paul is mystified by the Trump administration's actions.

"The U.S. claims the attack was in self-defense, that the Syrian jet was threatening U.S.-backed rebel forces in the area," he said in an emailed statement. "That claim has been debunked by even the pro-rebel Syrian Observatory for Human Rights."

The shoot-down of the Syrian plane is the latest in a pattern of military actions against the regime of President Bashar Assad.

"This marks the fourth recent U.S. attack on Syrian government forces as they engage ISIS in attempt to eject the terrorists from eastern Syria," Paul continued. "Each time, U.S. intervention has benefited ISIS."

Paul put forth similar criticisms in a June 16 column titled "Why are we attacking the Syrians who are fighting ISIS."

He said the Trump foreign policy visa vie the Middle East is barely recognizable from the non-interventionist platform upon which he campaigned.

"Just when you thought our Syria policy could not get any worse, last week it did," Paul writes. "The Syrian forces were pursuing ISIS in the area, but the U.S. attacked anyway.

"Why are we violating the sovereignty of Syria and attacking its military as they are fighting ISIS? Why does Washington claim that its primary mission in Syria is to defeat ISIS while taking military actions that benefit ISIS?"

Clare Lopez, senior vice president of research and analysis for the Washington-based Center for Security Policy, says Paul is justified in his concerns.

Lopez said the broader question is: "What is the core compelling U.S. national security objective that would plunge us into the middle of a 1,400-year-old intra-Islamic sectarian struggle between Shiites and Sunnis?"

Paul pointed out that President Trump appears dead set upon making sure Iran doesn't see its hand strengthened in the Middle East, even if that means putting a governor on the campaign to destroy ISIS.

"What is this really all about? Why does the U.S. military occupy this base inside Syria? It’s partly about preventing the Syrians and Iraqis from working together to fight ISIS, but I think it’s mostly about Iran," Paul wrote. "If the Syrians and Iraqis join up to fight ISIS with the help of Iranian-allied Shia militia, the U.S. believes it will strengthen Iran’s hand in the region. President Trump has recently returned from a trip to Saudi Arabia where he swore he would not allow that to happen."

But the intervention of the U.S. military in Syria comes with risks, the biggest one being direct engagement with the Russian military, Lopez said.

The administration appears to be poking the bear without a clear statement of objectives, she said.

Lopez wonders if the National Security Council and Pentagon have formulated a set of strategic objectives for this region of the Middle East along with the best course of action that would achieve those objectives.

"Or, are we sliding inexorably into a maelstrom that will chew up more American lives and treasure without having set out that overarching strategy first?" Lopez asks.

Even if the U.S. were to defeat ISIS, Lopez doubts it is prepared to defeat the idea that inspires ISIS, meaning the ideology.

"From the strategic level down to the tactical one, have the NSC/Pentagon actually conducted a thorough investigation of just who these rebel militia groups on the ground really are?" she asked.

Michael K. Nagata, former commander of American Special Operations forces in the Middle East, commented back in 2014: "We have not defeated the idea. We do not even understand the idea."

"Have we ever moved beyond that astonishing admission of dereliction of professional duty to know the enemy?" Lopez asked.

"Has the Trump administration conducted its own study of Islam, jihad and Shariah to a depth of being able to understand why either Shiite or Sunni groups are fighting each other, us, Western civilization?

"How does hurling the U.S. military into defense of what is very likely a jumble of Middle Eastern jihadist militias advance the strategic defense of the U.S. homeland, Constitution or core, compelling national security interests?"

In his email to WND, Paul said the situation with the Russians could easily escalate.

"We do know from the time of the Mujahideen in Afghanistan that elements in the U.S. national security establishment have utilized extremists in pursuit of their interventionist foreign policy goals. The fact that this strategy has failed – with tragic consequences – does not seem to faze proponents of this policy."

Paul said Russia is "furious" over the U.S. attack on the Syrian jet.

"They also had jets operating in the area, and the U.S. did not use the hotline designed to avoid any unintended disasters," Paul said. "Indeed, the use of the F-18 instead of a surface-to-air missile suggests that the U.S. understood the delicacy of the operation."