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How did START I reduce U.S. and Russian nuclear arsenals from their earlier levels?

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Strategic Arms Reduction Talks U.S. Pres. Barack Obama talking with Russian Pres. Dmitry Medvedev, just before the two leaders issued a joint statement about their continued cooperation on the New START Treaty, December 4, 2009.

The START I treaty significantly reduced the nuclear arsenals of the United States and Russia (and other former Soviet republics) from their previous levels of approximately 11,000 warheads on each side.1

The treaty implemented a phased reduction strategy to lower these stockpiles:1

First Phase: Limits were to be reached within three years of the treaty's implementation.1
Second Phase: By 1999, both nations were required to reduce their arsenals to a maximum of 7,950 warheads and 1,900 delivery vehicles (missiles and bombers). Within this limit, no more than 6,750 warheads could be mounted on deployed intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs).21
Third Phase: By the end of 2001, both sides further reduced their forces to 6,000 warheads and a maximum of 1,600 delivery vehicles, with no more than 4,900 warheads on deployed ICBMs and SLBMs.1

To ensure compliance, the treaty utilized rigorous verification measures, including on-site inspections, permanent monitors at the Votkinsk missile factory, and access to missile telemetry data.1