Why did START II never enter into force?

The START II treaty never entered into force due to a combination of political friction, legislative delays, and evolving security concerns between the United States and Russia.1

While the U.S. Senate ratified the treaty in 1996, the process in the Russian Duma moved significantly slower, causing the treaty to become a "hostage" to broader geopolitical tensions. Key factors included:1

• Geopolitical Irritation: The Russian Duma grew increasingly frustrated with Western policies regarding the Balkans and the Persian Gulf.1
• ABM Treaty Concerns: Russia became deeply concerned over American attitudes toward the Anti-Ballistic Missile (ABM) Treaty. By 2000, the Duma explicitly linked the fate of START II to the preservation of the ABM Treaty.1
• Resource Constraints: Russia preferred lower final force levels achieved at a slower pace, as it lacked the financial resources for the rapid decommissioning of weapons required by the treaty's original timeline.1
• Final Repudiation: Following the United States' withdrawal from the ABM Treaty in 2002, the Russian Duma formally repudiated START II.1