Americans in this era have proved themselves incapable of negotiating with each other regarding internal disputes at the national level. Thus, the genesis of the theory that a 'new' American civil war is brewing. Time and again Michael Vlahos and John Batchelor, who've been holding weekly discussions this year on the new civil war, have pointed out the obvious. Americans have lost the ability to compromise on domestic social issues that affect the entire U.S. nation. For this reason negotiation to resolve national domestic issues simply isn't possible at this time in American history.
The upshot has been 'winner take all' domestic politics where, as Michael noted during Friday's discussion (two-part podcasts here and here), only complete destruction or submission of the other side is considered acceptable.
So do not ask why Americans must no longer be involved in negotiations to resolve disputes between foreign governments.
Americans don't want to see compromises come from negotiations; they've repeatedly demonstrated they want to use their participation in negotiations between other governments to pressure all parties to submit to the 'American side.'
There's nothing wrong with U.S. foreign relations and defense policy that promote the American viewpoint. There's nothing wrong with the U.S. engaging in bilateral negotiations. There's plenty wrong with Americans pushing their view during multilateral negotiations where foreign governments must work out compromises to assure their mutual survival.
Americans must first learn to compromise with each other. Only then should they return to negotiations that are critical to entire societies outside American shores.