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The United States is prepared to double its defense budget if the Ukraine war spreads beyond the country's borders, chairman of the US Joint Chiefs of Staff General Mark Milley said on Thursday.

"The strategic end-state is that global rules-based international order that was put in place in 1945 was upheld… You achieve that end state when Ukraine remains a free, sovereign, independent country with its territory intact, and then you know that the rules-based [order] was upheld," Milley said.

"If that rules-based order - which is in its 80th year - if that goes out of the window, we will be very careful. We will be doubling our defense budget at that point," Milley said told the budget hearing.

Milley's statements come after the United Kingdom, a key ally of Ukraine's in light of the ongoing war, announced earlier in March an increase of $6 billion in the nation's defense budget over the next two years.

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The Kiel Institute for the World Economy reported in February that the US has officially overtaken Europe in assistance to Ukraine since the recent announcement of committing the staggering amount of 73.1 billion euros ($77.95 billion) in financial and military assistance to Kiev.

According to Joint Chiefs of Staff Chairman Mark Milley, the US military is assisting NATO partners and the Ukrainian government to defend the system that has avoided great power wars.

"We're supporting Ukraine and its fight to protect its sovereignty and supporting our NATO allies with the United States force presence in every single nation on NATO's eastern flank. This fight is not just in Ukraine's interest, it is in the US interest, to protect the system that has prevented great power war for eight decades," Milley said at a House budget hearing.

Additionally, as a result of Moscow's military operation in Ukraine, the US, the general peace on the European continent, and international stability are now under "acute" threat from Moscow, according to Milley.

The US Joint Chiefs of Staff Chairman considered the Ukrainian President's goal of retaking Crimea as a maximalist one that would be “extraordinary” to achieve militarily.

Mark Milley said both Russia and China have the means to challenge the interests and "the way of life" of the United States, adding that war with either Moscow or Beijing is not inevitable or imminent.