This is a VOA Special English Background Report.
Iraq's army has help from several groups of fighters loyal to President Saddam Hussein. These groups are also called militias or paramilitaries. They have been strongly resisting American and British troops in southern and central Iraq. They also work to suppress any uprisings by Iraqis.
Coalition officials say the strong resistance from these groups in southern and central Iraq has surprised them. They say they expected the irregulars to mostly stay near Baghdad to protect the capital.
Irregulars are fighters who do not belong to a group organized like a traditional -- or regular -- army. They often use methods of guerrilla warfare. One is to wear civilian clothes to carry out surprise attacks. Irregulars in Iraq have also dressed as soldiers and killed coalition troops during false surrenders.
Another method is to hide among civilian populations. On Tuesday, American Marines captured a hospital that had been used as a base in the city of An-Nasiriyah. Inside, they showed reporters large amounts of weapons and chemical protection equipment.
American military commanders say the fighters must be dealt with to stop the attacks against coalition troops. They say special operations and other forces will work to defeat these groups.
One group is called Fedayeen Saddam. One meaning for this name in Arabic is "those willing to sacrifice themselves for Saddam." The president's older son, Uday, established the group in the middle of the nineteen-nineties. Fedayeen members have killed and tortured Iraqis to suppress opposition.
Allied commanders say they will not permit such groups to stop the offensive toward Baghdad. They also say they will do all they can to avoid hurting civilians in the effort.
American and British forces must also deal with the regular Iraqi military. This includes the Republican Guard. Its thousands of soldiers are more trained and better equipped than other troops. There is also the Special Republican Guard, under Qusay Saddam, the president's younger son. Its job is to serve as a last line of defense around Baghdad.