Soviet concern about the issue grew with the U.S. development of highly accurate submarine-launched ballistic missile (SLBM) systems in the 1980s. Until then, the United States would have delivered most nuclear weapons by long-range bomber or ICBM. Earlier U.S. sub-launched missiles, such as the 1960s-vintage UGM-27 Polaris and 1970s-vintage UGM-73 Poseidon, were considered too inaccurate for a counterforce or first-strike attack, an attack against an opponent's weapons. SLBMs were reserved for attacking cities, where accuracy was of less importance. In the first case, an opponent with effective radar and satellite surveillance could expect a 30-minute warning of an attack before the first detonation. This made an effective first strike difficult, because the opponent would have time to launch on warning to reduce the risk of their forces being destroyed on the ground. The development of highly accurate SLBMs, such as the Trident C4 and, later, the D5, upset this balance. The Trident D5 is considered to be as accurate as any land-based ICBM. Therefore, US or UK Trident submarine systems could stealthily approach an enemy's coast and launch highly accurate warheads at close range, reducing the available warning to less than three minutes, making a counterforce first strike or a decapitation strike viable.