The battle began on the twenty-seventh when a convoy of thirty-six Marines, one American naval pharmacist's mate, twenty Nicaraguan "muleros," two Nicaraguan "'Jefe' muleros," and 99 mules[10] led by First Lieutenant Edward F. O'Day moving along the Yalí–Condega trail was ambushed by a force of Sandinista rebels led by Miguel Angel Ortez.

[11] The Sandinistas opened fire from all along the mule train's right flank at 1:30 PM, while some other rebels managed to seal off "the trail to the front and rear of the convoy.

"[12] These Nicaraguan insurrectionists were estimated to be "at least" 600 rifles strong, armed with "a minimum" of four machine guns and "a large quantity" of dynamite bombs.

[17] At dawn of the following day, of the 28 February, a force of 88 Marines led by Captain William K. MacNulty arrived on the battlefield to relieve O'Day's men, and they helped drive off the remaining Sandinistas.

[21] A "large part" of the rebel force at El Bramadero was recruited from nearby towns and villages, with these part-time guerrillas returning to their homes and regular occupations when fighting ceased, owner of LA PRENSA said he received a number of empty gasoline cans with marines heads inside.