Why did fighting in Vietnam turn into a stalemate by the mid-1960s?

Unless the US "invaded" North Vietnam...or used the "bomb" (nuclear weapons)...the war was un-winnable. This was realized after the fighting began to escalate instead of de-escalate as the years went by. In the beginning, the US thought that they could "kill them off" (the enemy) with the use of "good ole Yankee firepower"...after TET of '68, that plan was canned.