Ivica Olic put Croatia ahead after 11 minutes and Ivan Perisic seized on a poor clearance by Cameroon keeper Charles Itandje to race forward and drive in the second after 48. An unmarked Mario Mandzukic headed the third just after an hour and tapped in his second after 73 as Croatia strolled to a comfortable victory.
Cameroon, who have won only one game in five World Cups following their 1990 heroics, were reduced to 10 men five minutes before halftime when Song bizarrely elbowed Mandzukic in the back as the two man chased back in a benign situation on the halfway line and, without injured striker Samuel Eto'o, barely managed an attack all night.
Brazil and Mexico each have four points in Group A with Croatia, beaten 3-1 by the hosts in their first game, on three. Cameroon have zero points from two games. A win for Croatia against the Mexicans in Recife on Monday would put them into the last 16 for the first time since they reached the semi-finals in 1998.