The saboteurs managed to place four explosives on a Russian freight train carrying diesel and jet fuel, roughly 3,000 miles from the Ukrainian border. But more important than the destruction of the train, Ukrainian intelligence officials said, was the timing of the blast.

They needed it to blow up as the 50 rail cars were traveling through the nine-mile-long tunnel through the Severomuysky mountains, the longest train tunnel in Russia.

The Ukrainians were hoping to compromise a vital conduit for weapons being shipped to Russia from North Korea, at a moment when Ukrainian forces on the front are struggling to stave off relentless Russian assaults. Trains can be replaced and tracks quickly repaired. But serious damage to this tunnel, which took decades to build, might not be so easy to fix.

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But guerrilla tactics — including sabotage, commando raids, targeted assassinations and attempts to blow up ammunition depots, oil pipelines and railways — have taken on added importance as the two sides fail to make substantial advances at the front.

So at 5:20 p.m. on Nov. 29, a fire ripped through the tunnel, Russian Railways reported. Russian media broadcast footage of flames around the tunnel entrance, and officials said the explosion was caused by “the detonation of an unidentified explosive device.”

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“The war in Ukraine is changing right now, as Ukraine increases the number of guerrilla operations against Russian forces and decreases conventional operations,” said Seth G. Jones, an analyst at the Center for Strategic and International Studies who previously served as an adviser to the commanding general of the U.S. Special Operations Forces in Afghanistan. “The goal is to deliver death by a thousand cuts.”

Russia, with three times the population of Ukraine and a far larger military industrial complex, currently has the advantage in conventional warfare, especially with sustained Western military assistance for Ukraine in doubt. But military analysts point out that an occupying power is historically more vulnerable to attacks by saboteurs working for, or sympathetic to, the country under invasion. And the Kremlin’s scorched-earth campaign in Ukraine continues to fuel resistance in occupied territories.