BAGHDAD—Islamist insurgents seized control of Iraq's second-largest city on Tuesday in a brazen military operation that underscored the weakness of the central government across vast swaths of the country.

Prime Minister Nouri al-Maliki declared a nationwide "state of maximum preparedness" after government forces fled Mosul in disarray following four days of fighting. He asked parliament to declare a state of emergency. But he didn't say whether military forces were mobilizing to retake the city, 220 miles north...