The Lewis and Clark Expedition
The Catholic Church became corrupted, due to its overgrown power. Because of the Pope's overspendings, the Pope started the Sales of Indulgences. The Sales of Indulgences was a scheme to earn more money. It made people spend money so that they/family members go into heaven. This upsetted a reformer, Martin Luther. That started the Reformation. So in summery, the Catholic Church gained to much power ( more than the nobility) and it wanted more money. BTW, this only a little bit of it.
willing to go to war with Mexico was to gain new territory, thus strengthening his belief of the Manifest Destiny because he was a firm believer in Manifest Destiny