Regarding the US-Mexican War, President James K. Polk decided to request Congress for the authorization to engage in War with Mexico as a response to the attack of a US diplomatic mission sent to Texas by the Mexican Forces in 1845. As a congressman, I would vote in favor of this, as Mexicans were trying to invade territory that one dace before had already made it clear it did not want to belong to them. On the other hand, a hostile response received by the diplomatic mission is also a powerful reason.
Regarding the US invasion of Cuba in 1898, I would vote in favor of the action, as the US was already supporting a territory that clearly wanted its independence from Spain. Spain reacted violently, was it decided to engage in actions that would lead to the sinking of US battleship Maine. This was the turning point that would end convincing me, as a congressman, to vote in favor of the War against Spain.