Both Russia and Ukraine have been escalating their bloody conflict, now past the 1,000-day mark, seeking maximum advantage before Jan. 20, when President-elect Donald Trump will take office and probably bring a different U.S. policy toward the war. For Russia, escalation is about saving face — expelling Ukrainian forces from Russia’s Kursk region — and grabbing as much Ukrainian territory it can. For Ukraine, it’s a matter of surviving with its sovereignty and as much of its preinvasion territory as possible.

Escalation might be a hard concept to grasp in a savage struggle that has led to an estimated 1 million casualties. Russian President Vladimir Putin began raising the stakes this fall when he invited thousands of North Korean troops to augment his thinly stretched and decimated military. Ukraine responded with strikes inside Russian territory using British and American long-range missiles, albeit only after the Biden administration finally lifted its restriction on the use of such weaponry. The Russians have responded by firing a new medium-range, hypersonic ballistic missile — thankfully not armed with the nuclear warheads such missiles are capable of carrying — at a Ukrainian weapons factory in Dnipro. President Joe Biden has agreed to provide Ukraine with antipersonnel land mines, weapons with a record of causing civilian casualties, but also perhaps the only things that can help Ukraine hold its lines against Russian and North Korean infantry assaults.

The reason for this surge in fighting is clear: Mr. Trump seems inclined to strike a quick deal to end the conflict once he takes office. He has been vague about what an eventual deal would look like, promising only that he would swiftly solve the conflict upon taking office — or perhaps, somehow, even before. Vice President-elect JD Vance was more specific, saying an eventual settlement would probably entail a “demilitarized zone” that would include lands currently occupied by Russia. Hence the Kremlin’s drive to grab as much additional land as possible before Inauguration Day, and Ukraine’s desperate bid to resist it.

Ukraine’s European backers have shown signs of war-weariness and might be more amenable to a settlement. But the danger for Ukraine is that in his interest to strike a quick deal, Mr. Trump might settle for a bad one. A truce that ratifies Ukraine’s de facto dismemberment and leaves Ukrainians feeling disillusioned and betrayed by their Western backers would reward Mr. Putin’s aggression and encourage him to commit more of it.

Not only Mr. Putin. An abandonment of Ukraine — or a deal that leaves Ukraine untenably territorially diminished — would signal to dictators around the world that Western resolve comes with an expiration date. Imagine how Chinese President Xi Jinping would take a Western retreat from Ukraine as he contemplates taking Taiwan or the atolls and shoals in the oil-rich South China Sea. It’s not too soon to wonder — and worry — whether North Korea’s Kim Jong Un regards his army’s mission against Ukraine as preparation for a military move of his own on the Korean Peninsula.

The Ukrainians have fought valiantly but often with one hand tied behind their back. American and European assistance has been generous from the onset but often came late and with too many strings attached. First there was a reluctance to send M1 Abrams tanks, then to supply F-16 fighter jets, then to grant permission for using long-range missiles to strike deep into Russian territory. Mr. Biden eventually acquiesced in each case, but long after the weapons might have made a decisive difference on the battlefield. The fear each time was sparking an escalation with Russia. The intent seemed to be that Russia could not be permitted to win, but Ukraine would be given just enough not to lose.

A Ukraine left with a chunk of its eastern territory under Russian occupation is tantamount to a defeat — for Ukraine and for the West. During the presidential campaign, Mr. Trump made much of the Biden administration’s precipitous and ill-planned withdrawal from Afghanistan, which Mr. Trump said signaled to the world American weakness. An abandonment of Ukraine, after nearly three years of what has been a unified American and European front, would send the same sort of signal. And if it came as a result of Mr. Trump’s negotiated deal, the onus would be on him. He won’t have Mr. Biden to blame anymore.