"This will be a real review. It will be designed to ensure that NATO is moving fast and irreversibly toward Europe leading, stepping up to take primary responsibility for the defence of Europe," he told his NATO counterparts in Brussels on Thursday.
Hegseth lambasted European allies for failing to provide US forces access to bases in Europe to launch attacks on Iran, calling it "shameful".
"These allies, they put America's sons and daughters, our sons and daughters, at risk by denying them the predictable access, basing and overflight that never should have been in question at all," he said.
Taking the microphone at the top of the meeting, Hegseth also railed against migration and gender equality policies in Europe, in remarks reminiscent to those of Vice-President JD Vance in February 2025 that angered many Europeans.
"Instead of tanks and fighters and air defences, the focus has been on gender equity and climate change and defence austerity. Europe's borders flew wide open, welfare states expanded, defence budgets cratered. Along with Europe's belief in itself and its civilisation," Hegseth said.
Hegseth's comments largely mischaracterised European policies today.
European allies and Canada have launched an unprecedented effort to boost defence spending and expand their armed forces.
NATO Secretary-General Mark Rutte noted on Thursday that they spent $US90 billion ($A128 billion) more on defence in 2025, a 20 per cent increase over 2024.
And while Europe accepted large numbers of migrants and asylum seekers more than a decade ago, most countries have tightened their borders since.
The Trump administration now wanted a reboot of the 32-nation organisation to turn it into a "NATO 3.0" capable of deterring any threat, Hegseth said.
Hegseth's remarks came a few weeks after the US told its allies that it would no longer supply certain warships and aircraft if one of them comes under attack.
European allies and Canada are trying to work out how to plug the gaps.
NATO's supreme allied commander, an American, is working on back-up plans to defend Europe after the US signalled on June 3 that it would no longer supply an aircraft carrier and support ships, aerial refuelling planes and dozens of fighter jets, among other military assets, in a crisis.
The Trump administration insists that it needs to be able to plan for two simultaneous conflicts and wants more military resources at hand should a conflict break out with China in the Indo-Pacific region.
Under NATO's collective security guarantee - Article 5 of its founding treaty - the 32 allies pledge that an attack on one of them will be considered an attack on all.
It does not oblige them to provide military support, although many likely would.
In essence, the US is scaling back how it might help should an ally trigger Article 5.
The US does not intend to withdraw its nuclear weapons in Europe, which are key to NATO's deterrence.
To underscore that point, NATO's Nuclear Planning Group issued its first statement in 19 years after it had assembled at Thursday's meeting.
In the statement, it "recalled that the strategic nuclear forces of the alliance remain the supreme guarantee of allied security and underpin NATO's extended deterrence architecture".
The ministers "agreed to continue enhancing NATO's nuclear deterrence mission by modernising NATO's nuclear capabilities, strengthening its nuclear planning capacity, and adapting to achieve its security interests".
