Polk's wanted to provoke Mexicans to attack the U.S. Army so he could get Congress to declare war. This war was completely unjustified and since too many U.S. citizens were outraged and expecting action punishing mexico, the investigation of the actual accident was quick and faulty. This is obvious as it was later proven to be a mechanical error in the ships mechanical functions. The media also played a huge part, as you are probably familiar with yellow journalism, the media exaggerated and jumped to conclusions and spread negative conclusions of the even to the entire United States.