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

Updated: January 2009

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 makes 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 is due to expire on December 5, 2009, though the parties may agree to extend it for another five years or until it is superseded by another arms reduction agreement.

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 runs 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.