An agency whose domain includes internal security in Russia as well as espionage in the former Soviet states, the FSB has spent decades spying on Ukraine, attempting to co-opt its institutions, paying off officials and working to impede any perceived drift toward the West. No aspect of the FSB’s intelligence mission outside Russia was more important than burrowing into all levels of Ukrainian society.

And yet, the agency failed to incapacitate Ukraine’s government, foment any semblance of a pro-Russian groundswell or interrupt President Volodymyr Zelensky’s hold on power. Its analysts either did not fathom how forcefully Ukraine would respond, Ukrainian and Western officials said, or did understand but couldn’t or wouldn’t convey such sober assessments to Russian President Vladimir Putin.

The humiliations of Russia’s military have largely overshadowed the failures of the FSB and other intelligence agencies. But in some ways, these have been even more incomprehensible and consequential, officials said, underpinning nearly every Kremlin war decision.

“The Russians were wrong by a mile,” said a senior U.S. official with regular access to classified intelligence on Russia and its security services. “They set up an entire war effort to seize strategic objectives that were beyond their means,” the official said. “Russia’s mistake was really fundamental and strategic.”