Who were the Mexican-Americans fighting in the Mexican-American war?

They were fighting eachother over ownership of Texas.

Texas had previously belonged to Mexico.

"The Mexican–American War,[a] also known in the United States as the Mexican War and in Mexico as the Intervención Estadounidense en México (U.S. intervention in Mexico),[b] was an armed conflict between the United States and Mexico from 1846 to 1848. It followed the 1845 U.S. annexation of Texas, which Mexico still considered Mexican territory since the government did not recognize the Velasco treaty signed by Mexican General Antonio López de Santa Anna when he was a prisoner of the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its citizens wished to be annexed by the United States.[4] Domestic sectional politics in the U.S. were preventing annexation since Texas would have been a slave state, upsetting the balance of power between northern free states and southern slave states.[5] In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory in Oregon and Texas. Polk advocated expansion by either peaceful means or by armed force, with the 1845 annexation of Texas furthering that goal by peaceful means.[6] For Mexico, this was a provocation, but Polk went further, sending U.S. Army troops to the area; he also sent a diplomatic mission to Mexico to try to negotiate the sale of territory. U.S. troops' presence was provocative and designed to lure Mexico into starting the conflict, putting the onus on Mexico and allowing Polk to argue to Congress that a declaration of war should be issued.[7] Mexican forces attacked U.S. forces, and the United States Congress declared war.[8]".

https://en.wikipedia.org/wiki/Mexican%E2%80%93Amer... .