The Association of the U.S. Army is proud to mark the birthday of the nation’s oldest defense organization, the National Guard.    The National Guard celebrates its 378th birthday today, a stepping stone in U.S. history worth our attention because it reminds us of a fundamental American tradition of citizens organizing into military units for their common defense.   When the first National Guard drill was held in Salem, Mass., on Dec. 13th, 1636, it is unlikely anyone could imagine the role the Guard would come to play. The militia’s chief aim was protecting colonists in a new land. The Guard’s role in the colonies expanded and it has continued to grow over the centuries, but the concept of citizen-soldiers remains.   The U.S. has never fought a war where the National Guard wasn’t involved, there has never been a natural disaster where the Guard didn’t respond and there has never been a time from the very beginning of its existence when Guard members were not ready to answer the call.