WASHINGTON, March 29 — Senior American commanders and Pentagon officials are warning of an exodus of the military's most seasoned members of Special Operations to higher-paying civilian security jobs in places like Baghdad and Kabul, just as they are playing an increasingly pivotal role in combating terror and helping conduct nation-building operations worldwide. Senior enlisted members of the Army Green Berets or Navy Seals with 20 years or more experience now earn about $50,000 in base pay, and can retire with a $23,000 pension. But private security companies, whose services are in growing demand in Iraq and Afghanistan, are offering...