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https://www.perplexity.ai/search/how-was-icbm-with-nuclear-warh-9.SRJcGeQV6doGj_hFUrSw#0
how was icbm with nuclear warhead tested?

Answer
ICBMs with nuclear warheads have been tested in rare instances by a few nuclear-weapon states, but such tests have not been conducted since the 1960s. The United States, Russia (then Soviet Union), and China have performed such tests:

United States: On May 6, 1962, the US conducted "Frigate Bird," the only US test of an operational ballistic missile with a live warhead. It involved launching a Polaris A1 SLBM from the submarine USS Ethan Allen, which flew about 1,000 nautical miles before detonating a 600-kiloton nuclear warhead near Christmas Island in Kiribati.

Soviet Union: On September 13, 1961, the Soviets conducted Test 95, launching an SLBM from the Barents Sea to a test range on Novaya Zemlya. For safety, they used a reduced-yield warhead of about 6 kilotons.

China: On October 27, 1966, China conducted CHIC-4, launching a CSS-1 medium-range missile that flew about 900 kilometers before detonating a 12-kiloton warhead in the atmosphere at the Lop Nur test site.

It's important to note that these tests did not involve intercontinental-range missiles, but rather shorter-range ballistic missiles. Modern ICBM tests, such as the recent Minuteman III launch, use unarmed missiles with test reentry vehicles to assess reliability and accuracy without the risks associated with live nuclear warheads.