You choose to be the president you will have to manage your GDP use, you can buy military weapons and sign contracts, which if you are the one who starts it, the nation of which you want to sign the contract with has a chance to decline it, depending on the relation scale (0-100 with 100 being allies and 0 being at war) you can stage sieges, and prepare for war, war treaties are dictated by the winner, andyou can buy, manufacture, and research new technology, economics and weapons, Good luck.