A Jehovah's Witness today might work as a civilian employee in nearly any governmental agency. In Bible times, governments had few civilian agencies but rather administered nearly every governmental function through the use of military servicemen. Such functions as customs, law enforcement, and road construction were routinely assigned to the military.
In fact, John the Baptist specifically instructed seemingly godly men who were in military service that they should not "harass anybody" or embezzle, which would plainly imply that their work involved commerce rather than warfare.
.. ..(Luke 3:14) Also, those in military service would ask [John the Baptist]: “What shall we also do?” And he said to them: “Do not harass anybody or accuse anybody falsely, but be satisfied with your provisions.”
The situation is quite different today, and so there are many reasons why a true Christian (such as a Jehovah's Witness) would refrain from serving in the military of any country.
1. Bloodguilt by association
Most of the individuals in ancient Israel were not personally guilty of bloodguilt, yet their tolerance of it meant that most of them shared in bloodguilt by association. A true Christian makes it clear that he does not condone the bloodguilt of others.
.. ..(Isaiah 1:15-16) And when you spread out your palms, I [Jehovah God] hide my eyes from you. Even though you make many prayers, I am not listening; with bloodshed your very hands have become filled. Wash yourselves; make yourselves clean; remove the badness of your dealings from in front of my eyes; cease to do bad.
.. ..(Revelation 18:4) Get out of her, my people, if you do not want to share with her in her sins, and if you do not want to receive part of her plagues.
2. Use of violence
The Bible teaches that true worshippers would pursue peace and eschew violence. True worshippers do not even "learn war", which all military personnel must.
.. ..(Isaiah 2:4) They will have to beat their swords into plowshares and their spears into pruning shears. Nation will not lift up sword against nation, neither will they learn war anymore.
.. ..(Micah 4:3) And they will have to beat their swords into plowshares and their spears into pruning shears. They will not lift up sword, nation against nation, neither will they learn war anymore.
.. ..(Matthew 26:52) Jesus said to him: “Return your sword to its place, for all those who take the sword will perish by the sword.
.. ..(Luke 6:27-29) Continue to love your enemies, to do good to those hating you, to bless those cursing you, to pray for those who are insulting you. To him that strikes you on the one cheek, offer the other also
3. Allegiance
The Bible teaches that a Christian has only relative subjection to human governments. A true Christian cannot pledge his unfettered allegiance to anyone but God and Christ.
.. ..(Romans 13:1) The existing authorities stand placed in their relative positions by God
.. ..(Acts 5:29) We must obey God as ruler rather than men
4. Neutrality
A true Christian is a subject of God's Kingdom by Christ. Service in a country's military aligns an individual with that human government, a position at odds with the neutrality that is required of an ambassodor or envoy of Christ the King.
.. ..(John 18:36) Jesus answered: “My kingdom is no part of this world.
.. ..(Daniel 2:44) “And in the days of those kings the God of heaven will set up a kingdom that will never be brought to ruin. And the kingdom itself will not be passed on to any other people. It will crush and put an end to all these kingdoms, and it itself will stand to times indefinite
5. Coming judgment
.. ..(Revelation 6:15-17) And the kings of the earth and the top-ranking ones and the military commanders...keep saying to the mountains and to the rock-masses: “Fall over us and hide us from the face of the One seated on the throne and from the wrath of the Lamb, because the great day of their wrath has come, and who is able to stand?”
.. ..(Revelation 19:16-18) King of kings and Lord of lords. [After Armageddon] I saw also an angel standing in the sun, and he cried out with a loud voice and said to all the birds that fly in midheaven: “Come here...eat the fleshy parts of kings and the fleshy parts of military commanders and the fleshy parts of strong men