Below is the oath that one has to take to join in the United States military. Can a true believer  and follower of Jesus Christ rightly take this oath?

I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and  fidelity to any foreign prince, potentate, state, or sovereignty of whom or which I have  heretofore been a subject or citizen; that I will support and defend the Constitution and laws of  the United States of America against all enemies, foreign and domestic; that I will bear true  faith and allegiance to the same; that I will bear arms on behalf of the United States when  required by law; that I will perform noncombatant service in the Armed Forces of the United  States when required by the law; that I will perform work of national importance under civilian  direction when required by the law; and that I take this obligation freely without any mental  reservation or purpose of evasion; so help me God. In acknowledgement whereof I have  hereunto affixed my signature.

When a person takes this oath above, he is denying any allegiance to anyone or anything other  than the U. S. That includes our God, our Lord Jesus Christ(who is Prince, King and Potentate,  and sovereign above all) Going into the military, a person promises to have faith in U. S. and to  take up arms for it. God is the only one in whom we are to have faith/trust, not in man.

1Ti 6:12-15  Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called,  and hast professed a good profession before many witnesses.  13  I give thee charge in the  sight of God, who quickeneth all things, and before Christ Jesus, who before Pontius Pilate  witnessed a good confession;  14  That thou keep this commandment without spot,  unrebukeable, until the appearing of our Lord Jesus Christ:  15  Which in his times he shall  shew, who is the blessed and only Potentate, the King of kings, and Lord of lords

In Exodus 23:32-33 the Lord speaks to Israel of them not taking any covenants with the rest of  the world in which they live and why. Because their hearts will be turned away from God to  idols. And that is exactly what happened according to scripture throughout the Old Testament.

It is written  in 2 Cor 6:14-17 that we are not to be unequally yoked together with unbelievers  and why we are not, “for what fellowship hath righteousness with unrighteousness? and what  communion hath light with darkness?” And then exhorts us to, “Come out from among them,  and be ye separate, and touch not the unclean thing; and I will receive you….”

Jesus said in Matthew 5:33-37, Ye have heard that it hath been said by them of old time,  Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: But I say unto you,  Swear not at all; neither by heaven; for it is God’s throne: Nor by the earth; for it is his footstool:  neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head,  because thou canst not make one hair white or black. But let your communication be, Yea, yea;  Nay, nay: for whatsoever is more than these cometh of evil.” 

Every idle word that men speak, they will have to give an account of in the day of judgment  (Matthew 12:36-37)If we make an allegiance with the world(government or otherwise)then “Know ye not that the friendship of the world is enmity with God? whosoever therefore will be a  friend of the world is the enemy of God.” (James 4:4)

Mat 5:43-48  Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate  thine enemy.  44  But I say unto you, Love your enemies, bless them that curse you, do good to  them that hate you, and pray for them which despitefully use you, and persecute you;  45  That  ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the  evil and on the good, and sendeth rain on the just and on the unjust.  46  For if ye love them  which love you, what reward have ye? do not even the publicans the same?  47  And if ye  salute your brethren only, what do ye more than others? do not even the publicans so?  48  Be  ye therefore perfect, even as your Father which is in heaven is perfect.