Some people deny God's existence just because war and evil also exist. The idea is that if God is all-powerful, then He would never allow evil—like war—to exist. One might admire such idealism. But we can't admire the ignorance about the Bible that comes with the idea. The fact is that God allows war and evil to happen—for now, and for a purpose.
God to mankind: love, not war
All of human history and culture results from what happened in the Garden of Eden.
Adam and Eve are the original parents of everyone on earth. God created within them the ability to produce all the variations of the tribes and nations on earth today: "From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands" (Acts 17:26 Acts 17:26And has made of one blood all nations of men for to dwell on all the face of the earth, and has determined the times before appointed, and the bounds of their habitation;
American King James Version×, New International Version).
But God did not want them to make war on each other.
First God made Adam. Hours later on the same day, using one of Adam's ribs, He made Eve. Unlike the animals, they were made in God's image. From their first breath they were full grown, fully developed and able to speak—but with no clothes. Adam and Eve didn't have anything to hide from each other! God immediately performed the first wedding ceremony: "God blessed them and said to them, 'Be fruitful and multiply'" (Genesis 1:28 Genesis 1:28And God blessed them, and God said to them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moves on the earth.
American King James Version×).
God commanded them as husband and wife to make love, but He did not command them to make war.
When did war happen?
It so happened that there was a lying, cheating, stealing and deceiving snake hanging around the garden near the tree of the knowledge of good and evil (see Genesis 3).
The snake was actually the chief fallen angel or demon who had borne a distinction rendered in Latin as Lucifer—meaning "light-bringer" and signifying the "day star." He was ultimately given the label Satan, which in Hebrew means "adversary." He's also called the devil, from a parallel Greek term meaning "accuser" or "slanderer." He appeared as a talking serpent in the garden to tempt and deceive Adam and Eve.
The angels were made eons before man. God didn't force His way of goodness and love on them—nor does He on us. They had to choose to do good and reject evil—which most of them did. However, Lucifer didn't believe that good was the best way. He made himself the enemy of God (see Isaiah 14:12-15 Isaiah 14:12-15  How are you fallen from heaven, O Lucifer, son of the morning! how are you cut down to the ground, which did weaken the nations!
 For you have said in your heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also on the mount of the congregation, in the sides of the north:
 I will ascend above the heights of the clouds; I will be like the most High.
 Yet you shall be brought down to hell, to the sides of the pit.
American King James Version×). Then he led a rebellion of one third of the angels to also sin and choose evil as their way (see Revelation 12:4 Revelation 12:4And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
American King James Version×). They sealed their own fate. Together, they made war on God, and that was the beginning of war.
It was an insane war. The demons were outnumbered two to one in the angelic realm. And most foolish of all, they were fighting their Creator—Almighty God! It's impossible for the created to be more powerful than the Creator. What were they thinking?! Answer: They were thinking only of themselves—not caring about others or about God.
It sounds unbelievable, but in fact these unfaithful, demonic angels will attack God again. It's going to happen shortly before the return of Jesus Christ: "And war broke out in heaven: Michael [a faithful archangel] and his angels fought with the dragon [Satan]; and the dragon and his angels fought, but they did not prevail, nor was a place found for them in heaven any longer. So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him" (Revelation 12:7-9 Revelation 12:7-9  And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,
 And prevailed not; neither was their place found any more in heaven.
 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceives the whole world: he was cast out into the earth, and his angels were cast out with him.
American King James Version×).
The bad news is that they will also incite a human world war like we've never seen before.
Man and war
So how did man get involved in war and evil?
Back to the Garden of Eden: At the serpent's deceitful instigation, Eve and Adam ate from the tree of the knowledge of good and evil. God specifically commanded them not to eat from it. More than that, they didn't eat the fruit of the tree of life, which God commanded them to do (see Genesis 2:8-9 Genesis 2:8-9  And the LORD God planted a garden eastward in Eden; and there he put the man whom he had formed.
 And out of the ground made the LORD God to grow every tree that is pleasant to the sight, and good for food; the tree of life also in the middle of the garden, and the tree of knowledge of good and evil.
American King James Version×and 15-17). Those two trees symbolized the two great ways of life: good vs. evil, give vs. get. War comes from choosing evil to selfishly and violently get things from others (James 4:1-2 James 4:1-2  From where come wars and fights among you? come they not hence, even of your lusts that war in your members?  You lust, and have not: you kill, and desire to have, and cannot obtain: you fight and war, yet you have not, because you ask not.
American King James Version×). War is not God's way.
What about nations that fight wars to defend themselves or others from attackers? You can understand why some nations have fought "for the right" as they saw it. You can appreciate the service of those who risk their lives to protect their nations.
Although God offered to fight ancient Israel's wars for them, His model nation ultimately chose to fight its own wars and suffer the consequences. At times God used Israel to execute His judgment (discipline or punishment) on other sinful nations. At other times He disciplined Israel by bringing other nations against it because of national sins.
But none of this changes the fact that it was not God's desire from the beginning that human beings, His children, would fight and kill one another in warfare.
Adam and Eve cast the mold by choosing the devil's way over God's way. Because of that, their firstborn son Cain started the first human war. You can read all about it in Genesis chapter 4. In a jealous, religious rage (sound eerily familiar?) Cain premeditated, plotted, attacked and murdered his own brother Abel. When the population of the world was tiny compared to today, this was a major, catastrophic war.
God pleaded directly with Cain to reconsider and choose the way of peace: "Why are you angry? And why has your countenance fallen? If you do well, will you not be accepted? And if you do not do well, sin lies at the door" (Genesis 4:6-7 Genesis 4:6-7  And the LORD said to Cain, Why are you wroth? and why is your countenance fallen?
 If you do well, shall you not be accepted? and if you do not well, sin lies at the door. And to you shall be his desire, and you shall rule over him.
American King James Version×). Sin spawns war. Throughout history there is always another war. But there doesn't have to be.
Out-of-this-world peace plan
The time is coming soon when all nations will put aside their differences and finally seek to faithfully follow Jesus Christ. Even among those today who claim to follow Jesus, very few are really doing the things He says to do. If you want to be a true follower of Jesus Christ—and I hope you do—then here is His out-of-this-world peace plan for you and everyone else: "Jesus answered, 'My kingdom is not of this world. If My kingdom were of this world, My servants would fight, so that I should not be delivered to the Jews; but now My kingdom is not from here'" (John 18:36 John 18:36Jesus answered, My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence.
American King James Version×, emphasis added).
Those who want to truly follow Jesus Christ won't fight in the wars of this world. That doesn't mean they don't love their countries, but it does mean that they love and serve the Kingdom of God first and more than any human nation. They know that all people are God's children.
When Jesus returns to the earth He will stop all human wars and establish an incredible era of a totally new, warless human culture. It will be a time when "nation shall not lift up sword against nation, neither shall they learn war anymore" (Isaiah 2:4 Isaiah 2:4And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruning hooks: nation shall not lift up sword against nation, neither shall they learn war any more.
American King James Version×). The good news is that you can be part of that peace plan now!