A warmonger is someone strongly pro-war. Warmongers favor war above all other options.
Words with monger in them usually refer to people who sell or promote something, like how a fishmonger sells fish. A warmonger is someone who is constantly promoting war: they always want to invade or attack another country, and they’re very loud and persistent. This word is usually an insult, because it suggests the person wants war regardless of the cost and even if it isn’t the best option.