[1] Prior to the fall of Talca in early March, Juan Mackenna had been positioned along the rata Itata river with a small division of patriot troops, whilst Bernardo O'Higgins had remained with the main body of the army in Concepción.

After Talca fell to Royalist forces, however, Mackenna realised his lines of communication with O'Higgins were now cut and that he was isolated.

Mackenna, the colonel of engineers on the patriot General Staff, formed a solid defensive position, building three mutually supporting bastions on local hills on the north bank of the north side of the Itata river near Membrillar.

A misinterpreted order resulted in confused skirmishes breaking out between the forward units of both armies.

The Royalist forces made a disorganised assault on the Patriot trenches surrounding the field by the village of Membrillar; this attack was repelled and immediately the Royalist officer Manuel Barañao led a second attack, this time on the central positions held by Mackenna.