Why it was relatively easy for the forces to capture parts of Mexico north Rio Grande?

The statement is incorrect. In the Mexican-American War of 1846-1848, it was not relatively easy for the forces to capture parts of Mexico. The war lasted two years and was a challenging and bloody conflict for both sides. The American victory did not come effortlessly but involved complex military campaigns over extensive distance as Mexican troops fought staunchly defending their territory despite significant disadvantages in logistics and resources compared to the American military.