President Joe Biden has spent enormous diplomatic capital on countering Russian aggression toward Ukraine. 

His administration relentlessly broadcast doomsday warnings about an impending invasion - which proved to be correct - and declared that no less than the international order was at stake. 

But Mr Biden has also made clear that the Americans are not willing to fight, even though the Russians clearly are. Furthermore, he's ruled out sending forces into Ukraine to rescue US citizens, should it come to that. And he's actually pulled out troops who were serving in the country as military advisers and monitors. 

Why has he drawn this red line in the most consequential foreign policy crisis of his presidency? 

No national security interests 

First of all, Ukraine isn't in America's neighbourhood. It is not located on the US border. Nor does it host a US military base. It does not have strategic oil reserves, and it's not a major trade partner. 

But that lack of national interest hasn't stopped former presidents from expending blood and treasure on behalf of others in the past. In 1995 Bill Clinton intervened militarily in the war that followed the collapse of Yugoslavia. And in 2011 Barack Obama did the same in the Libyan civil war, both largely on humanitarian and human rights grounds.

In 1990 George H W Bush justified his international coalition to expel Iraq from Kuwait as defending the rule of law against the rule of the jungle. Biden's top national security officials have used similar language when describing Russia's threat to international principles of peace and security. But they have been preaching a response of economic warfare through crippling sanctions as a response, not military operations. 

Biden doesn't do military interventionism 

This has something to do with President Biden's non-interventionist instincts. 

Granted, they were developed over time. He supported US military action in the 1990s to deal with ethnic conflicts in the Balkans. And he voted for America's ill-fated invasion of Iraq in 2003. But since then he's become more wary of using US military power. 

He opposed Obama's intervention in Libya as well as his surge of troops in Afghanistan. He resolutely defends his order to withdraw US forces from Afghanistan last year despite the chaos that accompanied it and the humanitarian catastrophe left in its wake. 

And his top diplomat Antony Blinken - a Biden "Whisperer" who's crafted the president's foreign policy over some 20 years of working at his side - has defined national security to be more about combating climate change, fighting global diseases and competing with China than about military interventionism. 

Americans don't want a war either

A recent AP-NORC poll found 72% said the US should play a minor role in the Russia-Ukraine conflict, or none at all. 

They are focused on pocketbook issues, particularly rising inflation, something Biden has to be mindful of as mid-term elections loom. 

In Washington the crisis is consuming lawmakers on both sides of the aisle, who are demanding the toughest of sanctions. But even reliably hawkish voices like Republican Senator Ted Cruz don't want Biden to send American troops into Ukraine and "start a shooting war with Putin."

Another foreign policy hawk, Republican Senator Marco Rubio, has said war between the world's two largest nuclear powers would not be good for anybody. 

The danger of a superpower confrontation 

That's the bottom line - Putin's stockpile of nuclear warheads. 

Biden doesn't want to spark a "world war" by risking a direct clash between American and Russian troops in Ukraine and he's been open about that. 

"It's not like we're dealing with a terrorist organisation," the president told NBC earlier this month. "We're dealing with one of the largest armies in the world. This is a very difficult situation, and things could go crazy quickly."