How did the province of Sonsonate become part of El Salvador?

During the Spanish colonial period, the region west of the Lempa River—which the indigenous Pipil people called Izalcos—was organized in 1558 as the autonomous province of Sonsonate.12

While the province of San Salvador covered the eastern three-fourths of the territory that would eventually become the modern nation, Sonsonate remained distinct for centuries. It was not incorporated as a part of El Salvador until 1823.12