FORWARD OPERATING BASE SHANK, Afghanistan — The whirling sounds of huge helicopter blades piercing the fragile Afghanistan sky emanate a certain amount of power over a land torn by war since the late 1970s when the Soviet army invaded the country.Since late 2001 after terrorists hijacked four passenger jets — ramming two into the World Trade Center and one into the Pentagon, while crashing a fourth in a western Pennsylvania field after passengers overpowered the hijackers — the United States considered Afghanistan a breeding ground for terrorism.For the past 12 years, units have deployed and left Afghanistan, some numerous times …