Mexico fired first, they invaded the United States territory after slaughtering hundreds at the Alamo. They didn’t expect the United States to respond and certainly didn’t expect the border dispute to lead to the complete defeat of the Mexican military. What they didn’t know is the vast innovations developed at West Point that crushed all opposition, mostly through military engineering innovation.
While Mexico was completely defeated and President Pierce took advantage of the situation, it was the actions of Mexico that led to their defeat. They invited war and lost. The United States had every right to take territory but it decided to make it a legal transaction of land instead and led to peaceful diplomacy going forth.
The acquisition of the west thus doesn’t fall under conquest but a legal land transaction between governments. Same as the Louisiana Purchase, the Oregon Treaty, and the Alaska Purchase.