They cannot do what they want in the nation or to the nations of the world. They are limited by written Constitutional restrictions.

“Absolute monarchs will often make war when their nations are to get nothing by it, but for the purposes and objects merely personal, such as thirst for military glory, revenge for personal affronts, ambition, or private compacts to aggrandize or support their particular families or partisans. These and a variety of other motives, which affect only the mind of the sovereign, often lead him to engage in wars not sanctified by justice or the voice and interests of his people.”

-- John Jay, The Federalist No. 4