Answer:
Texas was originally a part of Mexico and had a greater population of its citizens in the area.
The most important set of actions which led to the war between US and Mexico include
U.S. Senate defeating the agreement of annexation with Texas
U.S. President John Tyler signing the Brown Resolution which offered Texas terms of annexation.
In response to passage of the Brown Resolution, Mexico's minister to the U.S., Juan N. Almonte, requests his passports, breaking off diplomatic relations between the two countries
Texas convention votes in favor of annexation to the United States
In Matamoros, Mexican General Francisco Mejia issues a proclamation threatening hostilities against U.S. troops
United States declares war against Mexico and US troops occupy Matamoros.