From the beginning, President Joe Biden has been emphatic: America will not engage directly in a conflict with Russia, which Biden has said would amount to World War III.

But even as Russian President Vladimir Putin surrounded Ukraine with nearly 200,000 troops in February, few imagined that his invasion would devolve into the brutality and atrocity that we're seeing reported from Ukraine every day.

Nevertheless, that's where we are, and there are no signs of an end to the bloodshed. Last Friday, a Russian missile attack was launched against an eastern Ukrainian train station packed with fleeing civilians. At least 57 were killed and many more injured.

Even the current best-case scenario is dreadful: If the war eventually grinds to an exhausted halt, with Putin controlling large portions of eastern and southern Ukraine and Crimea, the only way to get to that undesirable point is through the deaths of many, many thousands of Russians and Ukrainians, including many civilians, women, children and old people.

The difference between Putin and Hitler is a matter only of scope and scale, and the West's tolerance for this sort of brutal carnage cannot be unlimited.

Furthermore, eventually the stakes of the war in Ukraine may become more obvious and inescapable. Nowhere are those stakes better described than in Robert Kagan's 2018 book The Jungle Grows Back.

Kagan reminds us of how rare and fragile a thing the "liberal world order" is, the seven-decade reprieve from history as usual that began after World War II. According to Kagan, the liberal world order was largely engineered by the United States, and it resulted in a unique era of human history that values democracy, rule of law, equality, free speech, international cooperation over war and the rights and freedoms of individuals over the prerogatives of the state.

The desirability of these principles is so obvious that one might assume they would naturally thrive as humankind develops, but according to Kagan the liberal world order had to be imposed and maintained by force or, at least, by the threat of force. It's in constant danger of relapsing into the war, tyranny, disorder and poverty that has characterised the vast majority of human history.

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This more typical condition of humankind is what Kagan calls the jungle. The jungle is never completely subdued. The liberal world order is threatened constantly by its resurgence.

In short, first, there's nothing inevitable about the characteristics of the free world that provide the kind of life that we Americans value so highly. They are under constant threat in nations around the world, including in the U.S. The war in Ukraine is the current, violent instance that represents the larger battle.

Second, it's naive to believe that the battle between the free world and the autocracies of Russia, China, North Korea and Iran can be won without force. We may hope otherwise, but history has another story to tell.

All of this would be considerably simpler if the primary antagonists in this confrontation were not heavily armed with nuclear weapons. But the invention of the atomic bomb was always inevitable, and so was its use. The abolition of nuclear weapons is as unrealistic as the eradication of war itself.

A nuclear exchange has sometimes been called unthinkable. But in truth any nuclear power will use nuclear weapons if the circumstances reach a certain point of desperation.

Thus nuclear weapons are a significant element of the calculus that our adversaries use to advance their autocratic agendas. It's unfortunate, but nuclear weapons have to be part of the West's calculus, as well.

This is terribly risky, but it's in the nature of the sad point that world politics has reached. If we're unwilling to push back with a credible nuclear threat, the battle between a liberal world order and autocracy is already lost.