BAGHDAD — A chilling series of fatal attacks across Iraq on Monday sent a disheartening message to the Iraqi and American governments: After hundreds of billions of dollars spent since the United States invasion in 2003, and tens of thousands of lives lost, insurgents remain a potent and perhaps resurging threat to Iraqis and the American troops still in the country.

The 42 apparently coordinated attacks underscored the reality that few places in Iraq are safe. The number of American troops killed this year has jumped, ahead of their planned withdrawal. Monday’s strikes against civilians and security forces across the country made it the deadliest day of the year for Iraqis, and it came in many forms: suicide attacks, car bombs, homemade bombs and gunmen.

By sundown, when Iraqis broke their fast in observance of the holy month of Ramadan, the death toll had reached 89, including 3 suicide bombers, and an additional 315 people were wounded. The widespread and lethal nature of the attacks — compared with an average of 14 a day this year — frightened many Iraqis, because it suggested that radical Sunni insurgents, led by Al Qaeda in Mesopotamia, may have regained the capacity for the kind of violence that plagued Iraq at the height of the sectarian war in 2006 and 2007.