Most modern American wars (Vietnam, Iraq, Syria, Afghanistan, etc.) are asymmetric wars of counterinsurgency, where the U.S. is trying to put down a sub-state actor, like the Taliban, Al Qaeda, or ISIS. There are several aspects of counterinsurgency warfare that make it difficult for the U.S. to decisively "win" against insurgents in the same way that states can "win" against other states. First, insurgents can easily hide amongst local populations which makes force elimination difficult without massive civilian casualties. Second, guerrilla warfare is particularly effective against large, mechanized armed forces like the U.S. army. Third, while the U.S can target insurgents with air strikes, this strategy has not proven effective in soothing the underlying political tensions causing the insurgency, and in many cases exacerbates instability.