Why Is The Mexican War Failing

The Mexican war that took half of what Mexico had already like California, Texas, Nevada and Utah was all Mexico's fault in the first place? The Mexican war began on April 25, 1846 and it was all started on a bit of a misunderstanding towards the Mexicans and the U.S as they have both disagreed on the border of each other. One side decided on the Border being El Rio Grande and the other being on the Nueces River. Since The United States decided that the border was on El Rio Grande The U.S road a boat passing their decided border making the Mexicans attack and basically causing the war in the first place.

“Imbecil and distracted, Mexico never can exert any real government authority over such a country“ (O’Sullivan) Mexico was already failing in starting to sustain and help out there country. Since the Mexican government was failing in sustaining the U.S was helping out. “The Anglo saxon foot is already on California's borders marking its trail with schools and colleges, courts and representative halls, mills and making houses’ (O’Sullivan). Since the Mexican government at the time was failing the U.S was helping out when they were in need. Doesn't that show how the U.S was doing good and not bad and wanting peace instead of war?
The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte [Rio Grande]” Polk. Like i stated in the last paragraph the Mexican government was not in good condition and the U.S was helping them out by supplying them with need like education and buildings and the reason for that is because the U.S had already tried to make a peace with the mexicans in the past based on the quote. Peace offerings and helping them in need why would that be a good reason to attack the U.S even when they have offered you peace in the past and helped you out