HELSINKI — Proposals to impose cease-fires or territorial concessions to halt the war in Ukraine would serve only to legitimize President Vladimir Putin’s aggression and encourage future assaults, Secretary of State Antony Blinken said Friday as he laid out U.S. strategy for hardening Ukraine against long-term Russian threats.

Blinken, making what officials described as a signal address in the Finnish capital, made a case for what he said was Putin’s “strategic failure” in Ukraine, one he said had isolated Moscow, weakened its economy, and exposed the weakness of Russia’s once-feared military.