Well, I am entirely sure about what events led to the widening of the war effort, but the main differences from Vietnam to past wars were the strategies and overall differentness. Rather than trying to gain territory and hold ground until the enemy surrenders, the U.S rather fought in Search & Destroy missions, where scouts would report positions of North Vietnamese Army/Vietcong, and U.S helicopters full of infantry would fly in after a shelling of the position, after landing the infantry would disembark and scout the position and eliminate all hostiles in the area. If the U.S troops managed to kill all or most of the NVA/VC, they would mark it as a victory and leave the area, for more NVA/VC to likely return.
The American public did not rally behind the government because after so many wars and seeing what happened over there they decided that we need to "GET THE HELLicopters OUT OF VIETNAM". However I do not know why the veterans were so poorly treated after arrival back in the U.S.
that's all I can say.
