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START I at a Glance
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Contacts: Daryl Kimball, Executive Director, (202) 463-8270 x107

Updated: July 2017

START I was signed July 31, 1991, by the United States and the Soviet Union. Five months later, the Soviet Union dissolved, leaving four independent states in possession of strategic nuclear weapons: Russia, Belarus, Ukraine, and Kazakhstan. On May 23, 1992, the United States and the four nuclear-capable successor states to the Soviet Union signed the Lisbon Protocol, which made all five nations party to the START I agreement. START I entered into force December 5, 1994, when the five treaty parties exchanged instruments of ratification in Budapest. All treaty parties met the agreement's December 5, 2001 implementation deadline. START I expired on December 5, 2009.

Basic Terms:

  • 1,600 deployed intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and heavy bombers for each side.
  • 6,000 "accountable" warheads on ICBMs, SLBMs, and heavy bombers, of which no more than 4,900 may be on ICBMs and SLBMs, 1,540 on heavy missiles (the Soviet SS-18), and 1,100 on mobile ICBMs.
  • Ballistic missile throw-weight (lifting power) is limited to 3,600 metric tons on each side.

Counting Rules:

  • Heavy bombers equipped only with bombs or short-range attack missiles (SRAMs) are counted as carrying one warhead each.
  • U.S. heavy bombers may carry no more than 20 long-range air-launched cruise missiles (ALCMs) each. The first 150 of these bombers count as carrying only 10 ALCMs each.
  • Soviet heavy bombers may carry no more than 16 ALCMs each. The first 180 of these bombers count as carrying only eight ALCMs each.
  • No more than 1,250 warheads may be "downloaded" (removed from) and not counted on existing multiple-warhead ballistic missiles.

Other Provisions:

  • START I ran for 15 years with an option to extend for successive five-year periods. Based on commitments made at the March 1997 Helsinki Summit, the sides agreed in principle to negotiate an agreement making the START treaties unlimited in duration.
  • Separate "politically binding" agreements limit sea-launched cruise missiles with ranges above 600 kilometers to 880 for each side and the Soviet Backfire bomber to 500.

Posted: July 25, 2017