Six months after Russia launched its full-scale invasion of Ukraine, signs are accumulating that the balance on the military and economic battlefields is slowly tilting the way of Kyiv and its Western backers.

In the biggest war between European countries since World War II, the death and destruction have no end in sight. Ukraine is still struggling against Russia’s advantage in raw firepower, but the country’s defenders are increasingly hitting Russian logistics and bases, including in Crimea, as they receive more Western weapons.

A drone strike on the headquarters of Russia’s Black Sea Fleet in Crimea on Saturday was one of many recent signs that Russia’s rear areas are increasingly vulnerable to Ukrainian attack.

Political and popular backing for Ukraine in the U.S. and most of Europe remains robust, despite fears that a drawn-out war and rising energy and food prices could undermine Western unity.

The U.S., in particular, is sending Ukraine steadily growing quantities of advanced weapons such as the High Mobility Artillery Rocket System, or Himars, as well as crucial financial support. The U.S. announced another nearly $800 million in military assistance for Ukraine on Friday, including drones, artillery and ammunition. For the first time, the package includes mine-clearing equipment and tactical vehicles that suggest the U.S. is arming Ukraine in new ways to retake lost territory.

“The Russian military has lost much of what momentum it had and has redeployed a lot of its forces in anticipation of a Ukrainian offensive in the southern part of the country,” said Michael Kofman, director of Russia studies program at CNA, a defense research organization in Arlington, Va.

“I don’t think there is a natural stalemate on the ground,” he said. “I think there is at least another chapter to play out before winter.”

The outcome of that effort is far from clear, but the fate of the conflict now lies with what the Ukrainians are able to achieve.

Both sides are believed to have lost tens of thousands of soldiers killed or wounded since Moscow’s full-scale attack began on Feb. 24. Russia is struggling even more than Ukraine to replace losses of troops and materiel, relying on mercenaries, proxy militias and old tanks to fill the gaps. Russia’s economy is facing a far deeper recession than Western nations.

Some results of the war already seem settled. Russian President Vladimir Putin’s attempt to rewrite the ending of the Cold War by restoring Moscow’s historic sphere of influence in Eastern Europe has failed. His war on Ukraine has instead united almost all of Europe against him, revivifying the North Atlantic Treaty Organization, which is poised to add Sweden and Finland as new members.

Widespread evidence of alleged Russian atrocities against Ukrainians and Mr. Putin’s weaponization of energy, food and even nuclear safety have made his regime a pariah throughout the developed world—although not in the Global South, where skepticism toward the West runs deep.

And Ukraine has already achieved a political win by surviving as an independent country, against expectations in Moscow as well as many Western capitals that Kyiv would collapse under Russia’s onslaught. The war has reinforced Ukraine’s distinct national identity and its determination to reorient its economy, politics and security arrangements toward the West.

Ukrainian soldiers rest during shooting exercises.

Photo: Manu Brabo for The Wall Street Journal

Russian shells land on a village near Bakhmut, eastern Ukraine.

Photo: Manu Brabo for The Wall Street Journal

But the final outcome of the war remains as uncertain as its duration. Russia still has far more artillery and shells. The difficulty of advancing over open ground makes it hard for Ukraine to retake occupied land. Western military aid, especially from Europe, remains slow and stuttering from Kyiv’s perspective. Many Western policy makers continue to doubt that Ukraine can achieve military victory short of a level of Western support that might risk escalation into a direct war with Russia.

The Biden administration has been circumspect from the start, sending weapons in fits and starts and only providing more advanced capabilities such as Himars after weeks or months of careful consideration, fearing escalation or that equipment could fall into the wrong hands. The contemplative approach has opened the U.S. up to criticism that it didn’t move fast enough initially, even as U.S. officials contend they are getting materiel into Ukraine as fast as possible.

Ukraine’s badly damaged economy has begun to stabilize, but its government is acutely short of money, partly because the European Union hasn’t delivered on its promises of financial aid. Money-printing to pay for the war risks undermining Ukraine’s currency. 

And the harshest economic fallout won’t hit Europe until early 2023, when winter will test the EU’s frantic preparations for living without Russian gas.

Sandbags are piled up to protect a gas station in Kostyantynivka, eastern Ukraine.

Photo: Serhii Korovayny for The Wall Street Journal

It is normal for all sides to feel pain in a war of attrition, however. The question is which side can outlast the other and impose its will.

As summer ends, Ukraine’s defenders are showing a newfound ability to strike deep behind Russian lines, including in the Crimea and Kherson regions of Ukraine’s Russian-occupied south.

Russia’s offensive in the eastern Donbas area is losing steam. Moscow has been forced to redeploy its troops to shore up vulnerable positions in the south. Retaking large territories from Russian occupiers remains a formidable challenge for Ukraine’s soldiers, however.

“Ukraine has gained the strategic initiative. But we don’t know what they can do with it yet,” said François Heisbourg, a former French official and special adviser at the Paris-based Foundation for Strategic Research.

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Ukraine’s southern counteroffensive won’t be a mass frontal assault on Russian lines, Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelensky, said in an interview. Rather, he said, Ukraine will try to replicate the strategy it used to defend Kyiv: attacking Russian logistics behind the front line, including with guerrilla tactics, to degrade Russia’s ability to wage war and force a withdrawal.

“The Russians need ammunition, fuel and field headquarters that are close to the front. We destroy the fuel and ammunition, then there is confusion because there is no headquarters, so it is already a demoralized army. Then you start to strike and slice it up,” Mr. Podolyak said. “It worked in the defense of Kyiv, and it will work the same way in the counteroffensive.”

Mr. Podolyak said Ukraine needs more Himars and attack drones that can pierce Russian electronic-warfare defenses.

U.S. defense officials believe that while neither side is gaining significant ground against the other in the current phase of fighting, Ukrainian attacks on Russian infrastructure deep behind the front line show how the initiative has shifted.

Ukrainian soldiers hide from drones near the front line in Kherson, southern Ukraine.

Photo: Manu Brabo for The Wall Street Journal

Ukrainian soldiers prepare shells for a Western-supplied M777 Howitzer.

Photo: Joseph Sywenkyj for The Wall Street Journal

The war is entering a different phase compared with two months ago, when Russian forces had more momentum in the battle for Donbas, a senior Pentagon official said on Friday. “I would say that you are seeing a complete and total lack of progress by the Russians on the battlefield,” the official said.

Meanwhile, the EU remains on high alert about running out of energy this winter, though some say the risk of an outright natural-gas shortage is subsiding as countries buy up non-Russian gas and EU efforts to save energy and share supplies take effect.

Energy analysts say Europe’s outlook is less dire than it looked earlier this summer. Russia’s dramatic cut to gas deliveries to Germany via the Nord Stream pipeline, now operating at only 20% of its capacity, has forced the EU into action to buy enough liquefied natural gas and ensure it can reach all parts of the bloc. The EU is now racing against time to build LNG terminals in time for next spring, when the region’s gas reservoirs, which are now filling up, will be depleted again.

“We’re not forecasting gas shortages or electrical blackouts,” said James Huckstepp, European gas analyst at S&P Global Commodity Insights. But he said risks remain, including failure to complete new infrastructure in time, and the weather.

“Even if Putin cuts the much-reduced gas deliveries via Nord Stream to zero, we think Europe can get through winter, provided temperatures are normal,” said Mr. Huckstepp. But a combination of a total Russian cutoff and a particularly cold winter could force rationing for factories and households.

Even without rationing, economists expect the combination of high energy and food prices, rising interest rates and a global slowdown to push much of Europe into at least a shallow recession this winter. A sharper energy crunch would almost certainly spell a deep recession.

A lignite-fired power plant in Germany, a country whose energy supplies have been hit hard by the Ukraine war.

Photo: sascha steinbach/Shutterstock

Russia’s economic outlook is much worse than the West’s—although possibly less bad than predicted early this year, after Moscow stabilized the ruble and restored its oil exports. The International Monetary Fund now forecasts that Russian gross domestic product will contract 6% this year and shrink further for years to come.

A detailed Yale University study published in July looked at the state of Russia’s trade, industries and finances and concluded the country’s situation is more dramatic than the IMF forecast or Russian official data suggest, with the authors writing, “Business retreats and sanctions are catastrophically crippling the Russian economy.”

So far, Russia’s dwindling economic prospects haven’t moved Mr. Putin to end the war. Western officials say the goal of sanctions is to weaken his industrial and military capabilities, rather than to change his mind.

But Russia’s strategy of using economic pressure to undermine Western political support for Ukraine isn’t working either, so far.

Mr. Putin’s determination to continue the war and evidence of widespread alleged Russian atrocities against Ukrainians have left the EU with little choice but to continue to support Kyiv and sanction Moscow.

Public support for Ukraine remains high around Europe and North America. Despite anger over inflation and high energy bills, opinion polls show most Europeans aren’t blaming the problem on support for Ukraine. Calls to lift sanctions on Moscow remain largely limited to far-left and far-right politicians or figures with a history of pro-Russia sympathies.

Political divisions within the West have diminished since this spring, when leaders from France, Germany and Italy called for an early cease-fire in Ukraine. That infuriated countries in Northern and Eastern Europe that feel more exposed to Russian expansionism: Poland, the Baltic countries and others argued that a cease-fire that left Moscow occupying 20% of Ukraine would reward its aggression.

French President Emmanuel Macron faced particularly sharp criticism around Europe for his conciliatory rhetoric toward Mr. Putin, repeatedly saying Russia shouldn’t be humiliated.

Italian Prime Minister Mario Draghi, left, and French President Emmanuel Macron examined debris from attacks in Irpin, near Kyiv, during a visit in June.

Photo: Ludovic Marin/Associated Press

“The speed at which we were losing all credibility in the eastern half of Europe convinced Macron that it is not possible to play both sides,” said Mr. Heisbourg. Since a high-profile trip to Kyiv in June, Mr. Macron has swung more strongly behind the common NATO position of support for Ukraine.

The U.S. has pumped about $10.6 billion of military aid into Ukraine since the Biden administration entered office, with more assistance expected in the coming weeks. There is little sign that support for Ukraine is flagging in the U.S.

“Most Americans are sympathetic to Ukraine and Zelensky has become a folk hero in a sense with a very large percentage of the population,” said Larry Sabato, a political analyst at the University of Virginia. Sustaining large amounts of assistance might become harder if the war goes on for many years, he said.

Some analysts say there could be more opposition to support for Ukraine in Congress if Republicans gain control of one or both houses after the November midterm elections.

But a congressional staffer believes the Republicans won’t relent in their support.

“The votes will be there for Ukraine,” the staffer said in a text. “It’ll be a lot like the NATO accession vote for Finland and Sweden in the Senate, a lot of churn, but intellectually honest Republicans know what’s at stake in Ukraine.”

Many soldiers who died recently were buried at a cemetery near Kramatorsk, eastern Ukraine.

Photo: Serhii Korovayny for The Wall Street Journal

—James Marson contributed to this article.

Write to Marcus Walker at marcus.walker@wsj.com and Gordon Lubold at Gordon.Lubold@wsj.com