Washington (CNN) Donald Trump's assertion that the United States has "no choice" but to send 20,000 to 30,000 combat troops to fight ISIS in the Middle East raises a slew of complicated questions, military analysts said Friday.

It also represents an about-face.

In October, Trump spoke of potential perils.

"Everybody that's touched the Middle East, they've gotten bogged down," Trump said on NBC's "Meet the Press." "I don't want to see the United States get bogged down. We've spent now $2 trillion in Iraq, probably a trillion in Afghanistan. We're destroying our country."

At the CNN-hosted debate Thursday night, the Republican presidential front-runner sounded a different note.

"We really have no choice. We have to knock out ISIS," he said. "I would listen to the generals, but I'm hearing numbers of 20,000 to 30,000."

Ohio Gov. John Kasich took the same position on the fight against ISIS, saying, "You have to be on the ground."

And Texas Sen. Ted Cruz urged that "we need to put whatever ground power is needed."

But military analysts say sending U.S. troops to fight ISIS raises complex issues, some of them strategic, many political and others simply logistical.

More U.S. boots on the ground in Iraq would require Baghdad's openness to having them and a war-weary American public's willingness to send them. It would create the potential for confrontation with Iran-backed militias in Iraq or perhaps Russian troops in Syria.

It also would require complex decision-making about what exactly those 20,000 to 30,000 personnel would do and what kind of support they would need.

In the world of military planning, there's a 3-1 "tooth to tail" ratio -- for every soldier on the front lines, there are three people backing that fighter up with logistical and other support.

And then there's the question of whether sending U.S. troops would actually work.

"Unless the Iraqi government has the capacity to take the territory that's been liberated and govern the territory in a way that's acceptable to people living there, it won't last," said Michael Eisenstadt, director of the military and security studies program at the Washington Institute for Near East Policy.

He pointed to the experience after the U.S. surged troops in Iraq in 2007 only to have the Shiite-dominated government's sectarian policies leave Sunni populations alienated and ripe for supporting militant groups.

"You can't send 20, 30 thousand people in, wrap it up in just a few months and live happily ever after, because the fundamental problems remain," Eisenstadt said.

Trump and other Republican candidates were responding to a question about comments that Gen. Lloyd Austin, commander of U.S. Central Command, made on Tuesday when he told the Senate Armed Services Committee that more troops were needed to end the ISIS threat.

While that was interpreted to mean troops on the ground, CNN military analyst Mark Hertling said Austin's comments were consistent with the position U.S. military officials have long held -- that they would send added support for Iraqi troops entering the difficult battle to retake Mosul -- not combat troops.

"You're talking about battlefield enablers," Hertling said. "The intent is still for the Iraqi forces to lead the way on that."

Greater U.S. support for the Iraqis could come in the form of additional advisers, logistical help to move people and supplies, engineers to sustain roads ravaged by explosive devices and communications and aviation support.

"There will also be the need for increased intelligence operations to assist the Iraqi forces, to contribute to the targeting ability, overhead platforms like drones, which Iraqis don't have," Hertling said.

Austin told the senators that U.S. military personnel could help develop better intelligence on the ground, provide more advise-and-assist teams and help with logistics.

A crucial issue would be how much of that support the Iraqi government would be willing to accept. Iraq has opposed large numbers of U.S. combat troops on the ground and declined the support of Apache helicopters for a recent operation in Ramadi, according to Hertling, a retired Army general.

Another question is exactly how many troops would be enough.

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U.S. intelligence has at times estimated the range of ISIS forces as up to 30,000 fighters. With that number, the United States would want to send 90,000 combat forces to defeat them -- and those troops would need the corresponding "tooth-to-tail" ratio of people supporting them.

If a President Trump, Cruz or Kasich were to send tens of thousands of U.S. troops back into battle in Iraq, a domino chain of those logistical and support quandaries would have to be thought through.

To take an airfield or city and hold it, a military has to develop supply lines they can hold. As troops move forward, they need fuel, ammunition, water, protection from munitions that run the gamut from crude improvised explosive devices to chemical or biological weapons.

Then there are geopolitical questions to grapple with, Eisenstadt said, first among them the close relationship between Iran and Iraq.

"As far as we can tell, Iran has indicated that the return of American troops would violate their red line," said Eisenstadt, adding that popular mobilization units affiliated with Iran could once again start to attack U.S. soldiers.

"It sets us up for a potential confrontation with Iran or its proxy," he said.

And any foray into Syria to tackle ISIS there risks the possibility of an accidental clash with Russia.

"There is always the potential for miscalculation," Eisenstadt said.