What was the main reason that made United States and others go after Hitler?

The united states went to war after a German U-boat sunk a ship going from the UK to the US, which had American citizens in it. While the UK and France went to war after Germany attacked Poland, which incidentally was not the first country they had taken land from. Finally the soviet union joined the allies against Germany after Germany did not agree to giving certain concessions, and promptly attacked the soviet union.