Complex sentence: The remaining G and H models were used for nuclear standby ("alert") duty as part of the United States' nuclear triad, the combination of nuclear-armed land-based missiles, submarine-based missiles and manned bombers.
Split into simpler sentences: The remaining G and H models were used for nuclear standby ("alert") duty as part of the United States' nuclear triad. This triad was the combination of nuclear-armed land-based missiles, submarine-based missiles and manned bombers.