That was the same mission US troops had in Vietnam. U Any expansion of force is limited by supply lines (i.e. through Pakistan, Iran or Russia and that include AIR transportation) and NOT turning people into supporters of the Taliban by bombing wedding parties and other local gatherings (which tend to collect Taliban Soldiers at the same time).
In Vietnam, until Cam Ranh Bay base was completed, the US had a difficult time supplying troops in Vietnam, thus US sent in few tanks till AFTER 1968 when Cam Ranh Bay Base was completed (And why the Tet Offensive took place in 1968, even the Viet Cong realized the importance of the completion of that base).
https://en.wikipedia.org/wiki/Cam_Ranh_Bay
It was clear by 1968 that the the US Military could not shut off the supply lines from North Vietnam to the Viet Cong for the Viet Cong had popular support among the people along those supply lines and the US forces were NOT strong enough to patrol EVERY VILLAGE ALL OF THE TIME. The US Forces were to large to support and to small to put troops in every village. At the same time the Viet Cong were not strong enough to drive out US Forces but strong enough to force the US to keep forces at Platoon or large formation or find such forces being overrun by the people of Vietnam, organized by the Viet Cong in Militia unit (most of the time these were farmers, but when called into service by the Viet Cong effective soldiers).
Side note: Killing of the peasants of Vietnam for supporting the Viet Cong was also not an option, the people of Viet Nam depended on those farmers for the rice they ate, thus the US could not kill the peasants for supporting the Viet Cong for someone had to produce the food the South Vietnamese people eat.
The same thing with Afghanistan. During Vietnam, if you asked Americans who the people of Vietnam supported, American would say anyone but the Communists and give reasons why. On the other hand it was clear the people of Vietnam supported the Viet Cong and you could see this is who gained what ground when US or ARVN (Army of Republic of Vietnam, the name of the South Vietnamese Army) forces left, it was generally the Viet Cong NOT any pro south Vietnamese politician. The same thing when it comes to Afghanistan, when the US and its allies forces leave an Area it become Taliban (except along the border of Iran, where it becomes pro Iranian Afghan tribal area). Without US Military support, the Afghan Government would fall. That is how little support it has among the people of Afghanistan. If you ask Americans, they would say the Taliban has little support, but given the territory the Taliban control in Afghanistan and how fast the Taliban can move in once US and Ally forces withdraw from an area of Afghanistan, shows a lot of popular support for the Taliban in Afghanistan.
Thus the problem with Afghanistan is much like Vietnam. The US does not have the means to put down the Taliban but also has refused to leave the Taliban to take over Afghanistan. Given an inability to defeat the Taliban but a refusal to leave them win, the US policy has been since Bush's term of office NOT to permit the Taliban to win till after the end of the term of office of whoever is President (and that was the case in Vietnam, till Congress finally put its foot down and withdrew support for South Vietnam).
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