U.S. Russian efforts to limit nuclear forces largely have ignored their most fearsome components—the nuclear warheads. Arms control agreements have instead focused on limiting the number of deployed delivery vehicles and their launchers: ballistic missiles and their associated silos, mobile launchers or submarines; and long range bombers. START II limits the number of warheads that can be mounted on delivery vehicles, but is silent on non-deployed warheads. Presidents George Bush and Boris Yeltsin announced in 1991 that certain tactical warheads would be withdrawn and dismantled, but these initiatives were not legally binding and neither side could confirm that the promised reductions actually took place.