US Defence Secretary James Mattis has said competition between great powers, not terrorism, is now the main focus of America's national security, reports BBC.
The US faced "growing threats from revisionist powers as different as China and Russia", he said, unveiling the national defence strategy.
In an apparent reference to Russia, he warned against "threatening America's experiment in democracy".
"If you challenge us, it will be your longest and worst day," he warned.
America has been gripped by ongoing investigations into alleged collusion between the Trump 2016 election campaign and Russia.
Speaking in Washington, Mr Mattis also appealed to Congress to fund the military adequately and refrain from "indiscriminate and automatic cuts" to the US federal budget.
President Donald Trump is seeking to boost defence spending by 10%, or $54bn (£43bn), in his proposed budget plan for this year, and hopes to recoup that sum through deep cuts elsewhere, including to foreign aid.
This is the first time the defence policies of the Trump administration have been spelled out in one place.
The threats listed are the same as under the Obama administration but the order of priority is different.
Formerly, jihadist militant groups like Islamic State or al-Qaeda were the focus but latterly America's former Cold War opponents, Russia and China, have reasserted themselves strategically.
The three great nuclear powers have posed a permanent existential threat to each other since Cold War days.
The national defence strategy provides guidance for the 2019 defence budget.