The biggest conflict between European states since World War II has undergone swings of fortune that offer a reminder of war’s unpredictability. The failure of Russia’s early blitzkrieg fueled Ukrainian confidence that is ebbing as Russia concentrates its firepower on a narrower, grinding advance.

Marcus Walker, Daniel Michaels, Michael R. Gordon – Saturday WSJ

Russia’s War on Ukraine at 100 Days Has No End in Sight, Threatening Global Costs

On Friday, Russian forces advanced behind heavy artillery barrages in eastern Ukraine’s Donbas region, where they have slowly but steadily gained ground, sending tens of thousands of civilians fleeing westward.

Ukrainian President Volodymir Zelensky marked 100 days of war with a somber but defiant video message. “The armed forces of Ukraine are here,” he said. “Most importantly, our people–the people of our nation– are here. We have been defending our country for 100 days already. Victory will be ours! Glory to Ukraine!”

Many Western governments fear a destructive stalemate looms, with Russian President Vladimir Putin and Ukraine’s defenders locked in a struggle that is viewed as existential by both.

Around 6.9 million Ukrainians have left the country since the war began, according to the United Nations, with Poland alone receiving 3.7 million, although some are returning home. Millions more Ukrainians have been displaced internally by the Russian onslaught. The invasion has devastated cities in Ukraine’s east, including Mariupol, where at least 22,000 residents were killed during the weekslong Russian siege, according to local officials.

Ukrainian and international investigators are gathering evidence of possible war crimes in areas where Russian troops killed and mistreated civilians. Kyiv has accused Moscow of forcibly deporting large numbers of Ukrainians to Russia, including many children.

Mr. Zelensky said Thursday that Russia now controls 20% of his country’s territory. The problem for Kyiv—and for Western European governments proposing a cease-fire—is that Russia has seized much of the industrial heartlands of Ukraine’s east and vast tracts of its fertile agricultural land, while blocking Ukraine’s access to the sea, needed for exports.

That threatens to leave Ukraine as a barely viable state surviving on Western giving. Ukraine needs roughly $5 billion every month to cover essential government services and keep its battered economy functioning, officials in Kyiv have said, in addition to humanitarian aid and armaments.

Russia, meanwhile, faces a deep recession this year from Western sanctions and a long-term erosion of its economic potential. Absent an unexpected collapse by one side, a war of attrition looms that could steadily devour the resources of both countries.

The stakes are too high for Ukraine or Russia to back down. The war also threatens two long-accepted pillars of global order: The principle that territory can’t be annexed by force, and that the seas are free to all nations’ ships.

The war has made the world poorer. By driving up food and energy prices, it has complicated the troubled global recovery from the Covid-19 pandemic. The disruption of long-established energy and food supply relationships leaves much of the world facing a protracted and costly economic adaptation.

“The time of cheap fossil-fuel energy is over,” German economy minister Robert Habeck said recently.

Russia’s expansionism has brought the world’s advanced countries closer politically. But it has also exposed gaps in interests and outlooks between the West and the poorer global South, which has remained largely neutral, and where Russia’s narrative of anti-Western grievances—echoed by China—has many sympathizers.

With no outright Ukrainian victory in sight, the Biden administration has begun to emphasize that its goal is to increase Kyiv’s leverage for potential negotiations with Moscow.

“Most of these things end in some fashion diplomatically,” Secretary of State Antony Blinken said on Wednesday. “Unfortunately, the signs we’re seeing right now don’t suggest that Russia’s prepared to engage in a meaningful way in diplomacy.”

The war’s opening weeks, in which Ukrainian resistance far surpassed expectations and Russian military operations shocked the world with their disorganization, prompted visions of Ukrainian triumph and possibly even Mr. Putin’s downfall. Recent weeks have brought more sobriety, as Russian forces gradually capture territory by destroying settlements that might shelter opponents.

Russia started its invasion on Feb. 24 with rapid airborne and motorized assaults targeting Kyiv and other big cities in Ukraine’s north, combined with offensives on several other fronts. Moscow’s forces soon grew overextended. And Ukrainian fighters, supported by the local population, showed unexpected skill using high-tech small arms provided by the West, such as Javelin antitank rockets and Stinger shoulder-launched antiaircraft missiles.

Russian forces withdrew from northern Ukraine in early April after taking heavy losses and regrouped for a more concentrated attack in the east.

Now, in the open landscape of Donbas, Ukrainians can’t execute the same sort of skirmishing tactics that effectively defeated Moscow’s forces around Kyiv. Recent fighting has shown that large quantities of old-fashioned artillery can enable Russia to slowly grind out limited gains.

A relentless war of attrition has begun.

“The appetite of wars like this for people and munitions is voracious,” said David E. Johnson, a retired U.S. Army colonel now at Rand Corp.

Mr. Zelensky has begged Western governments for more and better weapons systems. The U.S. is obliging, though more slowly than Kyiv wants. Washington has sent Ukraine more than a hundred M777 howitzers and now will provide a precision-guided rocket system with a range of 48 miles—more than double the range of the artillery pieces.

Reflecting worries about escalating the conflict with Russia, the White House extracted a commitment from Mr. Zelensky that Ukraine wouldn’t use the missiles to target Russian territory.

U.S. Director of National Intelligence Avril Haines told Congress last month that Mr. Putin’s near-term goals now are to consolidate control of Donbas, try to envelop Ukraine’s best forces in the east and cement his hold on the land bridge from Crimea.

The Russian president hasn’t abandoned his longer-term goal to undermine Ukraine, she said, and he still appears to believe that he can win the test of will with the West.

“We assess that Putin’s strategic goals have probably not changed,” she said.

If neither side gains an advantage, “they are just going to keep going at it like dueling meat grinders,” said Chris Dougherty, a former U.S. Defense Department strategist now at the Center for a New American Security. As a precedent, he cited the Iran-Iraq war of the 1980s, “which settled into eight years of smashing each other in the face.”

Russia’s early failures and shift to brute-force tactics have shown its military to be less capable than the West feared—though still formidable in firepower.

The war has also shown Mr. Putin’s goals to be more radical, his appetite for risk-taking greater, and his predictability lower than was widely believed. Most of Europe saw him as a canny but cautious revisionist. Now, he is seen as Europe’s most aggressively revanchist leader since World War II.

“the West believed that Russia generally accepts the outcome of 1991,” when the Soviet Union collapsed after the Cold War and Moscow lost control of most of its satellite states in Europe’s east, said Ulrich Speck, a Berlin-based foreign-policy analyst. “But we can now see clearly that this is an openly revisionist leader who wants to roll back 1991.”

Thus the past 100 days have prompted a rethinking of Europe’s defense policies. Low military spenders such as Germany and Italy have vowed to reverse course. Finland and Sweden have applied to join the North Atlantic Treaty Organization. The U.K., still at odds with the European Union over the terms of Brexit, has moved closer to the continent on defense.

The conflict between Russian ambitions and Ukrainian national aspirations has historic roots, but has become steadily more acute this century as Mr. Putin has escalated efforts to reimpose Moscow’s control.

For Mr. Putin, who has long asserted that Russians and Ukrainians are one nation, the war is a last bid to re-establish Russia as a great power. Ukrainians, however, have united in their determination to escape Moscow’s influence and join democratic and prosperous Europe.

With Ukraine’s destiny at the heart of the conflict, proposals for cease-fires and territorial compromises from some Western politicians offer little appeal for either Kyiv or Moscow—at least while they are able to fight.

Ukraine hopes to take the offensive again in the coming months. Mr. Zelensky has said his goal is to battle the Russian army back to its prewar positions and then negotiate to restore the rest of Ukraine’s territorial integrity. Russia and its proxies have occupied Crimea and part of Donbas since 2014.

Leaders in Western Europe doubt Ukraine can realistically achieve that goal, short of much broader Western involvement in the war that they fear could lead to uncontrolled escalation and direct conflict with Russia.

Mr. Biden’s stated priority is to avoid war with Russia while assuring Ukraine’s independence and sovereignty. He has refrained from defining what victory would mean on the battlefield, saying only that Kyiv needs to determine that. Washington also wants sanctions, a strengthened NATO and reduced European energy dependence on Russia to limit Moscow’s scope for further aggression.

“Russia should have a lesser ability to repeat this exercise in the future,” Mr. Blinken said.

But fear of a long war is exposing the limits of Western unity. France and Germany are particularly skeptical about Ukrainian victory prospects and worried about the war’s economic fallout. Their diplomatic outreach to Mr. Putin seeking a cease-fire is fueling mistrust in Eastern European countries such as Poland that feel more directly threatened by an expansionist Russia.

“Would a compromise that gives the aggressor part of the territory bring stability? Or would it just be a way for Russia to have a break, recover from losses, and start a new round in two or three years,” said Mr. Speck.

However the war evolves, long-term economic losses for the whole world appear certain.

War, blockade and sanctions are pushing up food prices world-wide and hunger in poorer countries, particularly in Africa. African Union President Macky Sall told a gathering of EU leaders this week that fertilizer has tripled in price this year and is often unobtainable.

In the U.S. and EU, fuel prices have surged and inflation has hit 8%, a level not seen in decades and threatening political backlashes in countries just recovering from the economic shocks of the pandemic.

“This is a real learning experience in how supply chains get disrupted,” said Mr. Johnson at Rand. “Covid did a little, but nothing disrupts like a war,” he said.

Write to Marcus Walker at marcus.walker@wsj.com, Daniel Michaels at daniel.michaels@wsj.com and Michael R. Gordon at michael.gordon@wsj.com