It is considered controversial because of the 2 stances you can take on the topic. One is that the United States stole 40% of Mexicos land through the war influenced by the doctrine at the time “manifest destiny” Many say it’s stolen due to the US president at the time James K polk having ideas of manifest destiny. (Correct me if I’m wrong) but James had a big obsession over the land of California and wanted it badly. James pushed the US military to provoke a fight with Mexico in the disputed lands of Texas.
The other stand is that the United States obtained the land fair and square through the Mexican American war.Supporting this stand many would say that Texas after all did gain its independence from Mexico and wanted to be incorporated into the United States but Mexico provoked the US by not recognizing the annexation(corporation) of Texas as the 28 state of the United States. Many would also say that Mexican army were the ones to set of the first gun shots theoretically starting the Mexican American War.