WASHINGTON — Russian President Vladimir Putin’s war in Ukraine is going very badly. Within weeks of the February 24 invasion, Russia had failed in its prime objective of capturing its neighbor’s capital, Kiev, and subduing its government. And things have gone downhill ever since.

In response to Ukraine’s devastating counterattacks this month, which have reclaimed significant territory and destroyed or captured astonishing amounts of Russian materiel, Putin a week ago decreed a partial mobilization in Russia, calling for 300,000 military reserves to fight in Ukraine.

He has also hinted that Russia may use nuclear weapons, a threat that has been widely repeated in Russian state-sponsored media. Putin’s spear carrier, Dmitry Medvedev, Russia’s former president and prime minister and currently deputy chairman of its Security Council, said that Putin’s warning about using nuclear weapons is “not a bluff.”

This has obliged President Joe Biden to reply in clear terms. Jake Sullivan, his national security adviser, told reporters over the weekend that in high level contacts between the U.S. and Russian government, the Americans made it clear that Russia will suffer “catastrophic consequences” if it uses nuclear weapons.

Just what those consequences would be, however, Biden has not yet said, leaving a nervous rest of the world to ponder the worst scenario in a new war of words.

Both the administration and Congress have maintained unwavering support for Ukraine in supplying arms, training and intelligence. The U.S. has given Ukraine more than $15 billion in military assistance since the beginning of the Biden administration, and Congress unveiled a plan this week to offer $12.3 billion more in military and economic support.

This military assistance, from the U.S. and NATO and other allies, has played a decisive role in Ukraine’s spirited and deadly defense. However, determined to avoid “a third world war” in Europe, Biden has banned Ukraine from using weapons the U.S. has supplied to attack Russia’s home territory.

Nonetheless, weapons such as the U.S. HIMARS multiple launch rocket system have taken such a toll on Russian forces that the Russian media propagandists have begun describing the war as a conflict with NATO, which is bent on destroying Russia.

So a nervous globe awaits the next stage of the war, in which a desperate Russia may escalate in response to what its regime sees as an existential crisis.

Ever since the end of the second World War in 1945, when the German Nazi regime of Adolf Hitler was crushed by the collective arms of American and European allies, the threat of a nuclear war has kept the peace.

By mutual awareness that such a colossal calamity would be suicidal for all concerned, such an outcome has been avoided.

That hope remains at the core of rational thought everywhere, but it also rests on the condition that all world leaders conduct themselves accordingly. Putin’s latest actions and indications raise fear of his insecurity in the current situation that could lead to a misstep of colossal order for all concerned.