How costly would a war with Iran be for the U.S.? One 2020 presidential candidate says it would be "devastating."

U.S. Rep. Tulsi Gabbard, D-Hawaii, a U.S. military veteran who served in Iraq and Kuwait, laid out a bleak scenario for the U.S. if the country was to engage in military action against Iran.

"I know where this path leads us and I'm concerned because the American people don't seem to be prepared for how devastating and costly such a war would be," Gabbard said Thursday during an appearance on Fox News' "Tucker Carlson Tonight."

"I know where this path leads us and I'm concerned because the American people don't seem to be prepared for how devastating and costly such a war would be." — U.S. Rep. Tulsi Gabbard, D-Hawaii

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The 38-year-old member of the National Guard, who has been a member of the U.S. House since 2013 after serving as a Hawaii state lawmaker, told host Tucker Carlson that a war with Iran would embolden the Islamic State and other terrorist groups and put American military personnel at odds with the very militias they are training now.

"So, what we are facing is, essentially, a war that has no frontlines, total chaos, engulfs the whole region, is not contained within Iran or Iraq but would extend to Syria and Lebanon and Israel across the region, setting us up in a situation where, in Iraq, we lost over 4,000 of my brothers and sisters in uniform," Gabbard said. "A war with Iran would take far more American lives, it would cost more civilian lives across the region."

Gabbard also said a U.S.-Iran war would likely send millions of refugees fleeing into Europe, further destabilizing that continent.

"Not to speak of the fact that this would cost trillions of taxpayer dollars coming out of our pockets to go and pay for this endless war that begs the question as a soldier, what are we fighting for? What does victory look like? What is the mission?"

The Pentagon on Thursday presented proposals to the White House for sending military reinforcements to the Middle East to beef up defenses against Iran as tensions heighten in the region, Fox News learned.

Meanwhile, in comments aired by Iranian state-run media, Maj. Gen. Mohammad Hossein Baqeri, chief of staff of the Iranian Armed Forces, declared that the U.S. was an "arrogant and pharaonic threat" that invited "all Iranians to enter the fields of jihad, full alert, and the mobilization of more power."

During his conversation with Gabbard, Carlson asserted that President Trump is not interested in a war with Iran but national security adviser John Bolton "clearly is."

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Gabbard then criticized Bolton and Secretary of State Mike Pompeo and reiterated that a war with Iran would not benefit the U.S.

"How does a war with Iran serve the best interest of the American people of the United States? And the fact is it does not," Gabbard said. "It better serves the interest of people like [Israeli Prime Minister Benjamin] Bibi Netanyahu and Saudi Arabia who are trying to push us into this war with Iran."

Fox News' Gregg Re contributed to this report.