If the Jews had risen up in the early '30s, before the Nazis came to full power, and had killed all members of the Nazi party and any other known anti-Semites, then the Holocaust most likely would not have come to pass. However, in such a preemptive strike, who would have sympathy for the Jews? Although their actions would essentially have been in self-defense, they would probably have been seen as monsters for having killed far less people than the Nazis eventually came to kill. History judges what happened, however, not what would have happened.
The various tribes and people that lived in the land of Canaan before the Israelites eventually attained complete conquest of the land had some problems of various types that were a danger on numerous levels. Not to say that they were Nazis, but God needed to deal with them. As I said in an earlier exchange, there is probably no explanation that will convince some people who are just going to see mass killing as evil no matter what the reason is for doing it. I'll admit that it certainly strikes me as one of the more unsavory aspects of the Bible, but I do see the justification for Holy War.
Back in Genesis 15, God talks to Abram about the future of his descendants. He tells him that while He's going to give the land of Canaan to them, "the iniquity of the Amorites is not yet full." What does this mean? In some way that God has not explained, because the people of Canaan are quite evil, God intends to punish them, but He is giving them a last chance of about 500 years to clean up their act. Among several points I want to make about Holy War, the first is this: God never sends His people off to war without giving some sort of warning first.
Here in the book of Joshua, as I have already noted elsewhere, the people that the Israelites were coming in to the land to destroy knew about the plagues in Egypt. They knew about the crossing of the Red Sea and the drowning of the Egyptian army. They knew about a handful of battles that were fought during the wilderness wanderings. They knew about the crossing of the Jordan River. Whatever sort of warning God may have given them 500 years previously (which I speculate about, but don't know for sure), God has been sending a clear, urgent message for the last forty years to these people: God's vengeance is coming.
So one might ask, "What is the use of a warning like that?!" Well, compare the forty-year warning that was given to the Canaanites and went unheeded to the forty-day warning given to the people of Nineveh by Jonah: "Yet forty days, and Nineveh shall be overthrown." Not the most uplifting sermon ever preached, but it had an effect on the people of Nineveh that was highly disappointing to both Jonah and the SAB, apparently:
"So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them."As a result of this act of mass repentance, there did not follow an act of mass destruction. I suggest that it should be considered a great likelihood that if any of the cities of Canaan had reacted in the manner that Nineveh did, they would have been allowed to remain in the land. It worked for Rahab, after all. The fact is that while the God of the Bible is a vengeful God, He would rather forgive the repentant than inflict suffering on the unrepentant.
Another option that I think few people consider is the possibility that God would have allowed them to simply evacuate. After all, there were a few incidents in their wandering in the desert in which they went to war against a nation that simply wouldn't get out of the way. They would politely ask if they could just pass through on the way to Canaan, and not only were they denied passage, but the nation that they had made the request to decided to fight them just because they were there.
One of the main issues in respect to turning the land of Canaan into the nation of Israel was that the Israelites needed to have the land all to themselves. If they were to take the land, but leave the inhabitants in it, then they would have to deal with the mixing of their culture, which was a new culture that God was building from the ground up. While I once again admit that I don't know so much about ancient history, I am of the understanding that when Persia defeated Babylonia, Persia became very Babylonian; when Alexander conquered Persia, he adopted much of the Persian culture and integrated it into Greek culture; when the Romans overthrew the Greeks, the Greek culture and language remained dominant despite the fact of Roman rule. The Israelites needed to conquer Canaan without becoming Canaanites in the process, and really, the only was they could guarantee that was to make sure there was not a single living Canaanite left in the land.
God repeatedly tells the Israelites that they are not to allow the customs of the people of Canaan to be an influence on their own culture. These people were sexually immoral, practiced human sacrifice, worshiped idols, and were generally violent and unkind. Because the Israelites didn't completely cleanse the land of the former inhabitants, eventually they, too, fell into these practices, which is probably the biggest shame from the point of view that considers Joshua's Holy War a good thing, because in the end they killed thousands if not millions of Canaanites and still fell into the trap that Holy War had been designed to protect them from.
Another thing that Holy War is intended to protect the Israelites from is actually much more closely related to the musings I put forth at the beginning. Case in point: Israel's first King, Saul, was ordered by God to go to war against the Amalekites and totally destroy them. He almost did it, but the notable exception was that Saul took King Agag captive. After all, what difference could one guy make? Well, a few hundred years later, when the Israelites were subject to Persia, there was a man by the name of Haman who devised a plan to wipe out the Jews. Many people compare Haman to Hitler, but Haman wasn't German, he was an "Agagite", that is to say, a descendant of the man Saul failed to kill on God's orders. As I said, history does not judge what would have happened, but I strongly suspect God knows. Yes, Holy War is at times a matter of self-defense.
Something else to be considered as almost a side note, but I think an important one, is that God is not partial. The same way God gave the Amalekites about 500 years to shape up or ship out, He gave Israel 500 years (from the coronation of Saul to the Babylonian invasion was just a tad over 500 years). Eventually, after living in the land for a few centuries and failing to be the nation He wanted them to be, he sent the Babylonians and the Persians to carry them into captivity toward the east. There are some who have done far more extensive studies into the significance of the stretches of time that Israel was in the land and out of it; you might do a Google search on the subject to find a complicated theory on how a handful of verses in the Bible map out a complicated calculation that predicts the reinstatement of Israel as a nation in 1948, but it's out of my scope here. The main point is that while God gave the land of Canaan to the Israelites, and had them kill off the former inhabitants, it was with the understanding that they would strive to make Israel, to borrow a familiar phrase, "one nation, under God."
So yeah, Holy War is far from a beautiful thing, but my understanding is that God gave many opportunities for the people He set the Israelites on to avoid His wrath. They knew it was coming, they knew that they needed to repent, they had ample time to do it, but they freely chose instead to stand up to God and His people. God wins in the end, though. God always wins, and to refuse to accept that and put your lot in with the enemies of God is essentially suicide.
On top of all of this, something that even I myself had always missed when it comes to the book of Joshua is that the issue of Holy War has application in our own lives today, thousands of years later. The Bible makes it clear that God's wrath is still coming. God will eventually deal with all nations and all individual citizens of all nations. You have the choice to play the part of the Israelites in this scenario or play the part of the Canaanites. Are you going to clean out the sin in your life through the power of God, or are you going to go into the walled city of your pride, shut the doors and think, "When the wrath of God comes knocking, I'll just pretend I'm not here or something." Or maybe you'll be the sort who, when you hear God's people are passing by, will pick up your weapons and go out to attack them? Or maybe you'll be someone like Achan, who will march with the Israelites, but not be as devoted to the cause of righteousness as you appear to be? There are many parts to be played in this drama, a drama that gets played out daily in our modern world in a more abstract, spiritual manner. I hope you'll make the right choice.