WASHINGTON — A recent major exercise by the Russian military revealed significant strides in its ability to conduct the sort of complex, large-scale operations, using drones and other new technology, that would be part of any all-out war with the United States in Europe, according to American and allied officials.

Preliminary Pentagon and NATO assessments of the exercise, one of the largest of its kind since the end of the Cold War, are classified and will take months to complete. But Western officials said the military maneuvers, known as Zapad, Russian for “west,” far exceeded in scope and scale what Moscow had said it would conduct, and tracked more closely to what American intelligence officials suspected would unfold, based on Russian troop buildups in August.

Before the exercise, Russia said the drills would involve fewer than 13,000 troops engaged in a counterterrorism scenario in Belarus, the Russian exclave of Kaliningrad, the Baltic Sea region and around St. Petersburg. Instead, tens of thousands of Russian troops in the Arctic and Far East, the Black Sea, close to Ukraine’s borders and in the Abkhazia region of Georgia also joined in, Western military officials said.

“In effect, these activities together constituted a single strategic exercise, involving the full spectrum of Russian and Belarusian military,” said Oana Lungescu, the NATO spokeswoman. That array included warships, submarines, fighter jets, helicopters, tanks and artillery, air defenses, anti-ship missiles, special forces, and short-range and nuclear-capable intercontinental ballistic missiles.