SACRAMENTO, Calif. — The Bush administration has been unable to muster even half of the 2,500 National Guardsmen it planned to have on the Mexican border by the end of June. As of Thursday, the next-to-last day of the month, fewer than 1,000 troops were in place, according to military officials in the four border states of Texas, California, New Mexico and Arizona. President Bush’s plan called for all 50 states to send troops. But only 10 states _ including the four border states _ have signed commitments. Some state officials have argued that they cannot free up Guardsmen because of flooding in the East, wildfires in the West or the prospect of hurricanes in the South. ‘It’s not a combat priority. It is a volunteer mission,’ said Kristine Munn, spokeswoman for the National Guard Bureau, an arm of the Pentagon, ‘so it’s a question of balancing the needs of the Border Patrol with the needs of 54 states and territories, and all those balls roll in different directions.’ Bush’s plan for stemming illegal immigration by using National Guardsmen in a support role called for 2,500 troops to be on the border by June 30, […]