Afghanistan would be a good start. It’s hard to rank these things, but Afghanistan is definitely a contender for the most pointless and prolonged American war. From Doug Bandow at antiwar.com:
These days the U.S. is the world’s most militaristic power, threatening, droning, bombing, invading, and occupying far more countries than any other nation. Operating on the well-established principle that might makes right, American presidents routinely intervene with neither domestic nor international legal warrant, as in Syria today. Washington also routinely sanctions allies as well as adversaries, insisting that every person in every nation follow US dictates.
Today America is lawlessly engaged in low-level aggression against Syria. US troops have occupied much of the north, protecting the Kurdish zone known as Rojava, seized Syrian oilfields in the east, for both politics and profit, and cut Syrian roads, including the main route to Baghdad, Iraq – based solely on the president’s illegal orders. Imagine the Syrian army invading Montana, guarding a secessionist ministate, occupying the state’s shale oil fields, and blocking highways heading east toward Washington, D.C.
The US also is engaged in a dangerous minuet with Turkey, which occupies part of Rojava. Ankara views Syrian Kurds as a threat and twice invaded northern Syria, including last fall after pushing America aside. Turkey established the “Syrian Interim Government,” dominated by radical Islamist insurgents who have murdered and ethnically cleansed Kurdish inhabitants. The Pentagon admitted that Turkey “actively supports several hardline Islamist militias and groups ‘engaged in violent criminal activities’.”