People say the Soviet Union and United States lost in Afghanistan and Vietnam, respectively, because of asymmetric warfare. This is not true in either circumstance.
Russia succeeded in its goals in Afghanistan and left the country because they had more pressing concerns elsewhere. The pro-Soviet Afghan government faired very well for several years after the Soviet withdrawl in 1989, the Mujahideen had largely been crushed by 1986 when the Soviets began to leave, allowing the Mujahideen to return from Pakistan, and the Soviets learned valuable lessons from testing new field tactics. 1 million Afghans were killed, 2 million internally displaced, and 5 million fled to Pakistan and Persia, and all at the cost of 15,000 Soviet troops.
The United States never set out to win the war in Vietnam in the traditional sense. US goals involved defending the South from communist invasion. US forces could have invaded Hanoi, destroyed the DVR and left the VC to wither away after having the support kicked from under them, but Washington saw the risk of escalation with China as too great and decided to play small ball and eventually pull out, never having been defeated on the field.
The lessons learned from the Soviet-Afghan War and the Vietnam War can be applied to an hypothetical alien invasion of the earth to understand why asymmetrical warfare against extaterrestrials is doomed to fail.