The wide-ranging 45-minute speech comes as the military has faced controversy both at home and abroad, with President Donald Trump deploying troops in two Democratic-run US cities and ordering lethal strikes on small, alleged drug boats in the Caribbean.

Trump, who has overseen a rare purge of senior officers after taking office, has also ordered strikes on Iranian nuclear sites and Tehran-backed Yemeni rebels.

"This speech is about fixing decades of decay, some of it obvious, some of it hidden," Hegseth said, as he strode a stage in front of a massive American flag.

"Foolish and reckless political leaders set the wrong compass heading and we lost our way. We became the 'Woke Department.' But not anymore," he said.

Hegseth declared an end to "ideological garbage," citing concerns over climate change, bullying, "toxic" leaders and promotions based on race or gender as examples.

He also took aim at the Pentagon's inspector general -- which is investigating his conduct -- saying the office "has been weaponized, putting complainers, ideologues and poor performers in the driver's seat."

Trump was due to address the gathering of top officers later in the morning.

- Shakeups at Pentagon -

Amid speculation over reasons for gathering all the top brass in one place, Vice President JD Vance insisted it was "actually not unusual at all," and told reporters "it's odd that you guys have made it into such a big story."

The Pentagon only said last week that Hegseth "will be addressing his senior military leaders," and the lack of clarity over what would occur fed speculation that a major announcement.

In May, Hegseth ordered major cuts to the number of general and flag officers in the US military, including at least a 20 percent reduction in the number of active-duty four-star generals and admirals.

That came after the Pentagon announced in February that it aimed to reduce the number of its civilian employees by at least five percent.

Since beginning his second term in January, Trump has also purged top officers, including chairman of the Joint Chiefs of Staff general Charles "CQ" Brown, whom he fired without explanation in February.

Other senior officers dismissed this year include the heads of the Navy and Coast Guard, the leaders of the National Security Agency and the Defense Intelligence Agency, the vice chief of staff of the Air Force, a Navy admiral assigned to NATO, and three top military lawyers.

Hegseth defended the firings on Tuesday, saying: "it's nearly impossible to change a culture with the same people who helped create -- or even benefited from -- that culture."

US forces carried out a nearly two month-long campaign of strikes targeting Yemen's Iran-backed Huthi rebels earlier this year and also hit three nuclear sites that were a key part of Tehran's nuclear program.

And US troops have also been deployed in Los Angeles and Washington -- allegedly to combat civil unrest and crime -- while similar moves are planned for Portland, Memphis and potentially other cities.