This comes from a site of Catholic prayers, but you can adapt it to suit your religious system.
O Glorious Guardian Angel of the United States, to whom God has entrusted the care of our beloved country, we honor you and thank you for the care and protection you have given to this great nation from the first moment of its inception.
O Powerful Angel Guardian, whose watchful glance encompasses this vast land from shore to shore, we know that our sins have grieved you and marred the beauty of our heritage. Pray for us.
O Holy Angel, before the throne of God. Obtain for us, from the Queen of Heaven, the graces we need to overcome the forces of evil so rampant in our beloved land.
Help us, our God-given protector and friend, to respond wholeheartedly to the urgent pleas of the Mother of God…Assist us to offer the prayer and sacrifice necessary to bring peace and goodness to our nation.
We want to make you known and loved throughout our land, so that with your help we may become once more “a Nation under God”!