KYIV, Ukraine — For months Russian and Ukrainian soldiers have waged a brutal war across a 1,500-mile front line, inflicting casualties, fighting to the point of exhaustion and making slow gains in territory when they were not suffering costly setbacks.

After beginning with the Russian seizure of part of southern Ukraine and a failed strike at the capital, Kyiv, and then pivoting to a bloody artillery battle in the country’s east, the war is entering a third chapter. A battlefield stalemate prevails, with hostilities at a simmer, amid anxious uncertainty over whether — and when — Ukraine will launch a counteroffensive to try to break the deadlock.

The timing for any such attack has emerged as a pivotal strategic decision for Ukraine’s government. Both sides are preparing for a protracted war, but Ukraine has greater incentive to try to avoid it with potentially risky maneuvers as early as this fall — before the rainy season turns the countryside into impassable bogs, or energy shortages and soaring costs undermine European support.

“An offensive is risky,” said Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Virginia, assessing Ukraine’s options.

“If it fails, the outcome could affect external support,’’ he said. “On the other hand, Kyiv likely sees this as a window of opportunity, beyond which lies the uncertainty of a protracted war against a Russian army that has had time to entrench.”

From the Ukrainian perspective, the mostly static trench fighting cannot go on indefinitely. Leaving Russia in control of much of the southern coastline would cripple Ukraine’s economy, already cratering from the war and propped up by Western aid. It would also give space to Russia to solidify control in areas it has captured, blanketing news media and school curriculum with its propaganda, arresting or driving out opponents, and potentially declaring the land part of Russia after staging sham referendums.

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President Vladimir Putin is also facing some political pressure to secure a battlefield breakthrough — especially after Ukraine’s attacks on the Russian-occupied Crimean Peninsula, and the car bombing that killed an ultranationalist commentator last weekend. The attacks had Russia’s pro-war hawks calling for revenge.

But numerous signs suggest that Putin will ignore those calls and settle instead on a strategy of plodding offense designed to exhaust and kill Ukrainian forces. The latest evidence came on Thursday, when the Kremlin published Putin’s order increasing the target size of the armed forces by 137,000, to 1.15 million.

Analysts said the decree hinted that Putin was preparing for a long and grinding war, but not necessarily a large-scale draft that would mark a major escalation and perhaps prompt a domestic backlash.

“Expectations that this will end by Christmas or that this will end by next spring” are misguided, said Ruslan Pukhov, a defense analyst who runs the Center for Analysis of Strategies and Technologies, a privately owned think tank in Moscow. “I think this will last a very long time.”

Ukraine was bolstered this week by the promise of a $3 billion military aid package from the United States. Biden administration officials said the aid was as much a message to Putin that the United States is in this for the long haul, as it was to Ukraine that America will continue to try to hold the NATO alliance together in backing Kyiv indefinitely.

Administration officials insist that President Joe Biden is committed to helping Ukraine win, even in a war of attrition, if it comes to that. Colin Kahl, undersecretary of defense for policy, said at a news conference this week that Putin’s assumption that he can “win the long game’’ was “yet another Russian miscalculation.”

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More about Russia’s war on Ukraine

In Russian state media, the message that Russia might be only at the start of a long and existential war against the West — now being fought, by proxy, in Ukraine — is sounding with increasing clarity. It is a sharp shift from six months ago, when Ukrainians were depicted as lacking the will to fight and eagerly awaiting Russian “liberation.”

“We will have fewer Russian tourists in Europe, but the size of the Russian army will increase by 140,000 regular servicemen,” Igor Korotchenko, editor of a Russian military journal, said on a state television talk show. “I expect that this is just the beginning.”

While Putin may be content with a protracted standoff, President Volodymyr Zelenskyy of Ukraine is in some ways fighting against the clock.

“The very difficult state of our economy, the constant risks of air and missile attacks and the general fatigue of the population from the difficulties of war will work against Ukraine” over time, Andriy Zagorodnyuk, a former defense minister, wrote in the Ukrainska Pravda newspaper. He said the military should be prepared to advance, rather than defend.

“It makes no sense to drag out the war for years and compete to see who will run out of resources first,” he wrote.

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Stage-managed elections to justify annexation could come as early as next month, Western officials say, putting additional time pressure on Zelenskyy to launch an offensive.

But several military analysts say there is a disconnect between Ukrainian civilian leaders, pressing for a major victory, and military leaders who want to ensure they have sufficient troops and combat power before conducting a major offensive.

Over the last month, the Ukrainians have pivoted to the new strategy of so-called “deep war” — hitting targets far behind the front — after months of grim artillery duels and street fighting in the eastern region of Luhansk, which ultimately fell under Russian control by early July.

Using long-range, precision guided rockets provided by the United States and others, the Ukrainian military has been striking Russian weapons depots, bases, command centers and troop positions deep into occupied territory, including Crimea, the peninsula Putin seized in 2014.

Ukraine has for months been telegraphing plans for the major battle in the south; the types of weapons it has requested from Western allies and the tactics it pursues on the battlefield offering clues to its strategy.

Tellingly, a recent U.S. military assistance package included armored vehicles with mine-clearing attachments that would be used in a ground advance, suggesting preparations for the opening of what would be a new, ground attack phase of the war. Ukraine pushed back Russian forces that were in disarray in the battle for Kyiv last winter, but has yet to demonstrate it can overrun well-fortified Russian defenses.

With the decision on an attack in the south looming, Zelenskyy has taken pains to show unity with his generals. At a news conference this week, he praised the commander, Gen. Valeriy Zaluzhny, and denied rumors he intended to dismiss the general.

“We work as a team,” Zelenskyy said. Asked to assess the general’s performance, he said, “The most important assessment is we are holding on. That means the assessment is high. When we win, it will be the highest assessment.”