China civil war, pro US KMT lost.
Korea civil war, pro US S.Korea almost lost if it weren't for US interference.
Vietnam civil war, pro US S.vietnam lost.
Syria civil war, US backed rebellions lost.
Afghan civil war, the government army is collapsing.
Why? They had democracy or pursued democracy. Their troops were trained and armed by US. Why they sucked at war? Is US a curse? Even Germany and Japan have lost their martial spirit after they became US' allies. So is US an exception? US won some wars anyways. No. US would not win any war if it loses technology advantages. If a war between US and China happens, US will lose because technology gap between the two countries is not big enough..