WASHINGTON—The United States is walking a fine line as it shares vast amounts of classified intelligence with Ukraine, trying to help Kyiv defeat Russia’s invasion and avoid dragging the U.S. into direct conflict with Russian President Vladimir Putin, according to current and former American officials.

U.S. intelligence-sharing policy, they say, in essence comes down to this: Washington provides data on the movement of Russian troops, tanks and ships; Ukraine, which also has its own intelligence capabilities, decides when to take a shot.

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