Prior to the attacks against the United States on September 11, 2001, the National Guard's general policy regarding mobilization was that Guardsmen would be required to serve no more than one year cumulative on active duty (with no more than six months overseas) for each five years of regular drill. Due to strains placed on active duty units following the attacks, the possible mobilization time was increased to 18 months (with no more than one year overseas). Additional strains placed on military units as a result of the invasion of Iraq further increased the amount of time a Guardsman could be mobilized to 24 months.