3. How do we know we aren't going to be called to something like that today?
The key point in answering this question is made in the book by Gard. He points out that the God's people today – the church – are a theological entity, not a political one. There isn't a country now that is “God's country” more than another, so wars like the one in Joshua can't happen today.
This means, among other things, that fighting for God isn't physical fighting any more. Longman points to Ephesians 6.
Finally, be strong in the Lord and in his mighty power. Put on the full armor of God, so that you can take your stand against the devil’s schemes. For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace. In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. Take the helmet of salvation and the sword of the Spirit, which is the word of God.
And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people. Pray also for me, that whenever I speak, words may be given me so that I will fearlessly make known the mystery of the gospel, for which I am an ambassador in chains. Pray that I may declare it fearlessly, as I should.
(Ephesians 6:10-20, NIV)
Christian warfare is fought by praying, with weapons like truth, faith, righteousness and God's Word. It isn't about physical political battles any more, because God's people are a spiritual unity rather than a political one and our enemies are spiritual rather than physical.
And though there are battles in the book of Revelation, God's people don't fight them. When they win, they do so by speaking about Jesus, and not running away from death (Rev 12:11). The description of the great final battle is of a huge army coming against God's people to fight against them, “but fire came down from heaven and devoured them.” (Rev 20:9). God's people don't have to fight.
There's a different debate here about whether it's appropriate for Christians to serve in the armed forces. That's not the question here – the point is that “holy war” is no longer fought with the weapons of this world, because God's kingdom is not of this world.[intro | Part 1 | Part 2 | Part 3 | Part 4]