Hundreds of Ethiopian troops in armored vehicles rolled into Somalia on Thursday to protect their allies in this country's virtually powerless government from Islamic militants who control the capital.The move could give the U.S.-backed Somali government its only chance of curbing the Islamic militia's increasing power. But Ethiopia's incursion could also be just the provocation the militia needs to build public support for a guerrilla war. "We will declare jihad if the Ethiopian government refuses to withdraw their troops from Somalia," a top Islamic official, Sheik Sharif Sheik Ahmed, told The Associated Press.