The U.S. Army has sent aircraft to help Beltway police find the sniper who has murdered nine people. But a federal statute prohibits the military from enforcing domestic law. So, when, exactly, can soldiers help the cops?
Almost always, so long as they don't directly engage in police work. The Army can offer intelligence, transportation, and logistical assistance to cops, but it can't conduct searches or make arrests.
The law in question is a 124-year-old statute known as the Posse Comitatus Act, which makes it a crime for anyone in the Army or Air Force to enforce civilian law. (A "posse comitatus" is a group formed to enforce community law; think of an Old West posse.) Congress passed the act in 1878 as part of the compromise which ended the Reconstruction. The law was originally intended to prevent federal troops from enforcing Reconstruction-era race laws in the South.
No one has ever been prosecuted for violating the Posse Comitatus Act. That's partly because it has a number of exceptions—enough exceptions, it turns out, to virtually swallow the rule.
The Constitution, for example, says that the president has the duty to "take Care that the Laws be faithfully executed." That means he can call out the military to enforce Supreme Court decisions, just as Dwight Eisenhower did when he sent the 101st Airborne Division to enforce desegregation laws in Little Rock, Ark., in 1957. Congress has also given the president authority to suppress civil insurrections. In 1992, George H.W. Bush used this exception to send federal troops to quash the Los Angeles riots.
The law contains an implicit exception for the National Guard since it's in the employ of state governors. (Southern congressmen in 1878 were much less worried about state militias than federal troops, hence the historical exception.) Thus, governors have used Guard troops frequently, to respond to major natural disasters and to protect airports and bridges after Sept. 11.
In the 1980s, Congress mandated even more ways the military could provide support to local law enforcement. The main exceptions grew out of the "War on Drugs," allowing the military to do things like fly surveillance planes on the U.S. border with Mexico, train police officers in various military specialties, or provide radar data to the U.S. Border Patrol and Customs Service. A federal law also allows the military to share intelligence with local law enforcement officers, provided the intel is collected during normal military operations. Another major exception, created in 1996 after the Oklahoma City bombing, authorizes the military to provide support to local cops in the event of a chemical or biological attack.
The Army's current plan to fly reconnaissance planes in Maryland is covered by these exceptions: Army personnel will fly the planes and operate the equipment, but civilian police will ride along to analyze any evidence gathered while in flight.