BAGHDAD — Sunni insurgents led by an al-Qaida breakaway group expanded their offensive in a volatile western province on Saturday, capturing three strategic towns and the first border crossing with Syria to fall on the Iraqi side. It’s the latest blow against Prime Minister Nouri al-Maliki, who is fighting for his political life even as forces beyond his control are pushing the country toward a sectarian showdown. The towns of Qaim, Rawah and Anah are the first territory seized in predominantly Sunni Anbar province, west of Baghdad, since fighters from the Islamic State of Iraq and the Levant group overran the city of Fallujah and parts of the provincial capital of Ramadi earlier this year.The capture of Rawah on the Euphrates River and the nearby town of Anah appeared to be part of a march toward a key dam in the city of Haditha, which was built in 1986 and has a hydropower station that produces some 1,000 megawatts. Destruction of the dam would adversely impact the country’s electrical grid and cause major flooding.Iraqi military officials said anonymously that more than 2,000 troops were quickly dispatched to protect the dam. Rawah’s mayor, Hussein Ali al-Aujail, said the militants ransacked the town’s government offices and forced local army and police forces to pull out.