All present wars stem from the universal war that is waging between the Creator, Jehovah God, and the rebellious angel Satan the Devil. That war has been waging for thousands of years and has humankind as the focal point and smack dab in the middle. It has to do with God's universal Sovereignty, and his right to rule.
The angel, spirit son of God, we only know as Satan the Devil rebelled against God's rule, and he took humankind with him, getting the first human pair, Adam and Eve, to side with him, and as a result most of their offspring. He also got 1/3 of the heavens, spirit sons of God, angels, to rebel with him. So Satan has quite the rebellion going.
War would not be necessary if it weren't for that rebellion. But since its inception "man has dominated man to his harm" (Ecclesiastes 8:9).
In Daniel we are given a small glimpse into the powers that rule over earth in Satan's invisible control. There were are told:
"Do you know why I have come to you? Now I will go back to fight with the prince of Persia. When I leave, the prince of Greece will come. However, I will tell you the things recorded in the writings of truth. There is no one strongly supporting me in these things but Miʹcha·el, your prince."-Daniel 10:20-21.
These princes were demons, and it is shown here that they held positions over the respective realms. So it is really demonic spirits, the fallen angels who control the world, and rule the different nations behind the scenes. That is why the Christian is told:
"We have a struggle, not against blood and flesh, but against the governments, against the authorities, against the world rulers of this darkness, against the wicked spirit forces in the heavenly places."-Ephesians 6:12.
Human wars are not just of human origin, but have at their root cause Satan the Devil. Their wars are not justified.
But God is justified in waging war against the rebels of his good Sovereignty. So not all war is unjust. Unfortunately, God has to become a "manly person of war" in order to defend truth and righteousness and justice.
So all human war is unjust and wrong. A Christian does not get involved in the wars of this earth because they know that "the whole world is lying in the power of the wicked one." (1 John 5:19). And all human rule is in the jurisdiction, the perview of Satan the Devil, and in opposition to Jehovah God, and his reigning son in heaven Jesus Christ.
Holy Scripture depicts the holy and glorified Jesus Christ as an angel riding a white horse that rides forth to wage war in rightouesness:
"I saw heaven opened, and look! a white horse. And the one seated on it is called Faithful and True, and he judges and carries on war in righteousness. His eyes are a fiery flame, and on his head are many diadems. He has a name written that no one knows but he himself, and he is clothed with an outer garment stained with blood, and he is called by the name The Word of God. Also, the armies in heaven were following him on white horses, and they were clothed in white, clean, fine linen. And out of his mouth protrudes a sharp, long sword with which to strike the nations, and he will shepherd them with a rod of iron. Moreover, he treads the winepress of the fury of the wrath of God the Almighty. On his outer garment, yes, on his thigh, he has a name written, King of kings and Lord of lords."-Revelation 19:11-16
He is from the spirit realm and will wage war with both wicked humanity, including all governments now in existence, and the power behind human rule, Satan and his wicked demons:
“In the days of those kings the God of heaven will set up a kingdom that will never be destroyed. And this kingdom will not be passed on to any other people. It will crush and put an end to all these kingdoms, and it alone will stand forever."-Daniel 2:44.
Because Jesus is the King of kings and Lord of lords, and ruler over all God has granted him domain, he will in his righteousness and indignation wage war on wicked humankind and vanquish all evil. And God's government will alone remain and bring about worldwide peace and security for all meek and righteous humans wherein they will live forever in paradise on earth.
So all human war is wrong. But not all war is wrong. Righteous war carried on by Jehovah God through his son Jesus Christ is necessary. We are told not to avenge ourselves. But God does avenge. He is a God of mercy, but he is also a God of vengeance. And he will bring swift retribution and vengeance upon the wicked by means of Jesus Christ and his myriads of holy angels.
"This takes into account that it is righteous on God’s part to repay tribulation to those who make tribulation for you. But you who suffer tribulation will be given relief along with us at the revelation of the Lord Jesus from heaven with his powerful angels in a flaming fire, as he brings vengeance on those who do not know God and those who do not obey the good news about our Lord Jesus. These very ones will undergo the judicial punishment of everlasting destruction from before the Lord and from the glory of his strength."-2 Thessalonians 1:6-9.
Humans need not be considered, nor should they be involved when that war arrives. Many who know Jehovah's Witnesses know they are peaceful people who do not get involved with the wars of the world, mistakenly believing they are pacifists. That is not the case. They realize that Satan is the ruler of the world, and that to partake in the wars and the politics of the world, they take Satan's side in the universal war being waged. They remain neutral in earthly politics because they look to Jesus Christ as God's reigning king in heaven, and to his government as the only solution to humankind's pressing problems. They have full confidence that Jesus Christ will wage the war of Armageddon, and that will be the ultimate battle that will bring an end to vast masses of wicked humanity, and all human rule and will establish a permanent heavenly government over all the earth that will bring untold blessings to the obedient, peaceful, meek ones of the earth.
"The great day of Jehovah is near!
It is near and it is approaching very quickly!
The sound of the day of Jehovah is bitter.
There a warrior cries out.
That day is a day of fury,
A day of distress and anguish,
A day of storm and desolation,
A day of darkness and gloom,
A day of clouds and thick gloom,
A day of the horn and of the battle cry,
Against the fortified cities and against the high corner towers.
I will cause distress to mankind,
And they will walk like blind men,
Because it is against Jehovah they have sinned.
Their blood will be poured out like dust,
And their flesh like the dung.
Neither their silver nor their gold will be able to save them in the day of Jehovah’s fury;
For by the fire of his zeal the whole earth will be consumed,
Because he will make an extermination, indeed a terrible one, of all the inhabitants of the earth.”
-Zephaniah 1:14-18.