That's because it's like being a doctor trying to eradicate cancer that is spread throughout the patient's body in minute amounts. You can do a precision strike

That's because it's like being a doctor trying to eradicate cancer that is spread throughout the patient's body in minute amounts. You can do a precision strike against one small area, but how do you eradicate all the cancer without killing healthy cells (also in this analogy killing any healthy cells makes you a murderer). The only way to win is to convince the patient's immune system (the host country's civilians) to kill its own cancer.