sdb777 That order was strengthened a few times and now is EO 12333.
No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination.
There are also UN resolutions and international laws and treaties against assassinations. When we were not at war with Afghanistan and we struck OBL training camp in 1998 with cruise missiles; Once the executive order was given the 12333 was no longer in effect. It was deemed a terrorist training facility. So war time law applied. The actions of striking first were justified as self defense. People were killed there but it was not assassination by US military definition.
The 1998 missile strike was in retaliation for the group being responsible for bombing 2 US embassies and to thwart future plans. The base qualified as a military target, legally treachery was not implied, so any deaths resulting from the strike could not be deemed assassinations.
Same applied a few days ago to OBL again. Financing terrorist training and arms, rallying the jihadists to strike, his role as commander, involvement in coordinating and funding terrorist attacks all made him a military target. Any means necessary was legally justified according to US military wartime law. Though we are technically at war with Al Quaeda, it was not necessary. EO could have been given at peacetime with no legal retribution. It was also considered self defense of our nation. Just as many other attempted and successful missions have been employed since 1976.