The ten year (and counting) War On Terror has been unique among American wars of the last century. It has lasted longer and killed fewer Americans than any other major war since 1911.
During the last decade, about eight million American served in the armed forces, about the same number that served during the eight year Vietnam War. But twice as many troops served in a combat zone during Vietnam, and more than nine times as many troops were killed. Five times as many Americans were killed in Vietnam, if you count the civilian dead lost on September 11, 2001. But the big difference was that more people went to war during Vietnam, and many more were killed. Thus during Vietnam, about 1.2 percent of Americans served in the combat zone, compared to about .5 percent during the War On Terror.
World War II was much more different still, with six percent of Americans serving in a combat zone. Over all, during World War II four percent of the eight million men and women who served overseas were killed in action (291,557), .7 percent died from other causes, such as accidents and disease (113,842). In Vietnam, 1.8 percent of those in the combat zone died, while during the War On Terror, one half of one percent of those in the combat zone died.
But there was a downside to the lower death rate during the War On Terror. It meant that more troops spent more time in combat. During World War II, most troops spent less than a year in combat. While the war had been going on since 1939, most American troops did not get into combat until June 1944, and the war in Europe was over by the following May. The fighting in the Pacific was more sporadic, with many months of no contact between periods of intense combat. In Vietnam (and Korea) there was a 13 month "tour of duty" policy, and most troops served only one tour. But during the War On Terror, multiple tours were more common, because, unlike Korea and Vietnam, there were no conscripts. All the troops were volunteers, and most served at least two tours. This increased the incidence of combat fatigue (to PTSD, for Post Traumatic Stress Disorder).
The War On Terror is not a unique experience in American military history. Several multi-decade conflicts against irregulars have occurred in the past (Haiti in the early 20th century and the Indian Wars of the late 19th century, for example). Both of these conflicts resulted in lower American (and enemy) casualties than the War On Terror. The religious radical angle was absent in these earlier wars, which is why there have been more deaths in the War On Terror. But as with those earlier conflicts, U.S. troops developed new weapons and tactics to deal with the situation. It should be noted that, although the U.S. Marines were in Haiti for nearly 30 years (from 1914), the country still reverted to dictatorship and poverty when the marines left. This exposes a truth that many refuse to acknowledge. Fixing countries isn't easy. The "civil society" that we in the West take for granted, cannot just be conjured up. The harmonious relationships that enable some democracies to work, are not a given. Those relationships often require a lot of bad habits to be changed. This is not easy. Just check a history book. Winning wars is a lot easier than making lasting change is dysfunctional cultures.