WASHINGTON, Nov 11 (Reuters) - President-elect Donald Trump has picked Republican Representative Mike Waltz to be his national security adviser, two sources familiar with the matter told Reuters on Monday, tapping a retired Army Green Beret who has been a leading critic of China.

Waltz, a Trump loyalist who also served in the National Guard as a colonel, has criticized Chinese activity in the Asia-Pacific and has voiced the need for the United States to be ready for a potential conflict in the region.

The national security adviser is a powerful role, which does not require Senate confirmation. Waltz will be responsible for briefing Trump on key national security issues and coordinating with different agencies.

While slamming the Biden administration for a disastrous withdrawal from Afghanistan in 2021, Waltz has publicly praised Trump's foreign policy views.

"Disruptors are often not nice ... frankly our national security establishment and certainly a lot of people that are dug into bad old habits in the Pentagon need that disruption," Waltz said during an event earlier this year.

"Donald Trump is that disruptor," he said.

Waltz has a long history in Washington's political circles.

He was a defense policy director for defense secretaries Donald Rumsfeld and Robert Gates and was elected to Congress in 2018. He is the chairman of the House Armed Services subcommittee overseeing military logistics and also on the select committee on intelligence.

Waltz is also on the Republicans' China Task Force and has argued the U.S. military is not as prepared as it needs to be if there is conflict in the Indo-Pacific region.

In a book published earlier this year titled "Hard Truths: Think and Lead Like a Green Beret," Waltz laid out a five-part strategy to preventing war with China, including arming Taiwan faster, re-assuring allies in the Pacific, and modernizing planes and ships.

On Ukraine, Waltz has said his views have evolved. After Russia's 2022 invasion of Ukraine, he called for the Biden administration to provide more weapons to Kyiv to help them push back Russian forces.

But during an event last month, Waltz said there had to be a reassessment of the United States' aims in Ukraine.

"Is it in America's interest, are we going to put in the time, the treasure, the resources that we need in the Pacific right now badly?" Waltz asked.

Waltz has praised Trump for pushing NATO allies to spend more on defense, but unlike the president-elect has not suggested the United States pull out of the alliance.

"Look we can be allies and friends and have tough conversations," Waltz said last month.

Waltz demonstrated his loyalty to Trump earlier this year when he appeared at Trump's May 16 hush-money court hearing in Manhattan, one of only a handful of lawmakers to do so.

WHAT IT MIGHT MEAN FOR THE MILITARY?

Waltz, who was in the running to be defense secretary, could rankle some in the uniformed ranks. As a congressman from Florida, he has been at the forefront of a conservative movement opposed to teaching certain theories about racism and has criticized military officials for it.

He has also lamented the Pentagon's failure to fire generals and civilian bureaucrats who fail to perform.

"We need to get a culture of accountability into that place. No one ever seems to get fired, with these massive cost over-runs, massive waste," Waltz told Fox News last week. "We don't need managers there. We need reformers."

Trump is expected to have a far darker view of his military leaders in his second term, after facing Pentagon resistance over everything from his skepticism toward NATO to his readiness to deploy troops to quell protests on U.S. streets.

He has criticized the military for being political under President Joe Biden's administration.

In 2023, Waltz introduced an act in Congress that would have required an audit of "unnecessary and political DEI (diversity, equity and inclusion) programs" in the military and "restore a merit-based culture to our ranks."

During a tense congressional hearing with the top U.S. general in 2021, Waltz was critical of the U.S. Military Academy at West Point for teaching critical race theory, which maintains that legacies of slavery and segregation have created an uneven playing field for Black Americans.

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Reporting by Phil Stewart, Steve Holland and Idrees Ali; Additional reporting by Jasper Ward; Editing by Eric Beech and Chris Reese