Why did ancient Israel have an army? Was it because they didn't have faith? Did they believe in themselves instead of trusting God? Did they insist on fighting their own battles, rather than let God wage war for them?
Why did God fight for Israel in Egypt? Did they exercise faith then? Did they somehow lose their confidence in wilderness, after having seen God's mighty miracles? Does that make sense? They'd heard what God was going to do to Egypt from onset, and they saw with their own eyes what God did through 10 consecutive plagues. After defeating world's only superpower, had God spent His energy?
God took on gods of Egypt single-handedly. He didn't want Israel's participation lest their deliverance be misconstrued as a successful slave revolt. He took a slave people to become His Servant-Nation. Israel had only changed masters.
When God led Israel out of Egypt by Moses, He purposely told Moses to bypass hot spots where Israel would encounter hostile forces, lest they opt for "security" of Egypt (Ex. 13:17). They went out of their way to avoid war! It doesn't sound like they were eager to do battle when you read accounts of their griping and complaining to Moses (Ex. 14:10-12).
Moses, full of faith, encouraged congregation to believe God. The first time Israel engaged in battle was at Moses' command! He instructed Joshua (also faithful) to assemble a fighting force and defeat pesky Amalekites (Ex. 17:8-16).
God had fought first few rounds against Israel's enemies, showing Israel He would destroy their foes. Now God would fight in and through and for Israel! Moses' uplifted hands of blessing and authority insured Israel's military success. It was evident to all where their true strength came from (Ps. 33:16). Israel was only a fighting instrument in God's Hands (Isa. 41:15, Micah 4:13).
Many people understand commandment against killing should be translated "murder." God commands His ministers of state to execute judgment and utilize capital punishment for various offenses (Rom. 13:4). We're told to remember Moses' law and obey it (Malachi 4:4). God has authorized State to act within guidelines of His Law Book.
Israel wasn't anxious to enter Canaan - they were afraid! Moses admitted they were outnumbered and outgunned by Canaanites, but reassured Israel that GOD would be with them and grant them supernatural victory (Deut. 7:1-2, 16-24). When spies brought their report to Moses about Canaan's men and might, they wanted to run with their tails tucked between their legs (Numbers 13:31). Only Joshua and Caleb put things in perspective and encouraged people to trust in God (Numbers 14:9). Israel didn't have any faith in their own ability or God's!
What many have failed to realize is ancient Israel wasn't going to enter Promised Land by faith alone; it was going to require faith and works! God wasn't going to do all work for them. Israel had to actively claim their inheritance. Don't we also?
Israel lacked faith that GOD could work miracles through them! They knew God could take on their enemies, but they doubted He could work through them. They concentrated on their weakness rather than on God's strength. God expected Israel to fight His battles and He would guarantee their success. Israel took one city at a time, slowly but surely making progress, subduing land and defeating their foes, and yet giving God all credit - like our process of conversion (Ps. 44:1-8).
Even David, a man after God's own heart, fought numerous battles for God and His people, but considered God Victor. David would ask God whether he should attack at this time or place, and what to do, when and how, and God answered him (1 Chron. 14:10).
Did Abraham, father of faithful, sin by waging war against those who had kidnapped his relatives (Gen. 14)? Melchizedek blessed him, He didn't rebuke him! Levi and sons, following Moses' call to arms (at God's command), went through Israel's camp and slaughtered those who'd sinned with golden calf (Ex. 32:28). Did they lack faith? No! They were acting faithfully! They put God before family and friends. Couldn't God have just sent a plague? Yes. But He worked through men.
Since God works through men, why didn't Jesus let Peter defend Him? 1) Because prophecy had to be fulfilled and 2) because God's Kingdom wasn't established yet - otherwise Jesus said His servants should fight! This example teaches us that loyal citizens ought to be willing to fight and defend their king and country!
Presently we have a "dual citizenship." Paul didn't deny that he was a Roman citizen or hesitate to exercise his civil rights (Acts 16:37, 22:25, 25:11). He didn't just turn other cheek when he was about to be scourged! There's a time and a place to wage war or declare peace (Ecc. 3:8); to suffer persecution and death, or to run for your life ( Mat. 24:16, John 8:59)!
Spiritually speaking, we should all understand we're only mortal, taking up temporary residence on earth, and therefore be mindful of eternity and live accordingly, waiting for our eternal inheritance in God's Kingdom (2 Cor. 5:1, Ps. 39:12, 2 Pet. 3:11).
But what about "spiritual fornication?" If we exercise our right to vote and get involved in politics, isn't that defiling ourselves with this world's system? Only if and when it contradicts commandments of God. We should never disobey God to follow men (Acts 5:29). We're in this world and do take an active part in its system of government and education everytime we go to work and pay taxes or send our kids to school or college! We're only to avoid those aspects of this world's system that are Biblically wrong (1 Jn. 2:16).