I disagree almost entirely with Daniel.
There may be justifications for nations going to war with one another, but there is never justification for God's Church to participate in them. Wars are a sign of man's inability to love man, man's quest for power, and misguided aspects of patriotism. Whether being invaded or not, lives are worth more, on both sides, than what flag flies above.
Remember that Jesus lived when Israel was occupied by what the Jews saw as the most evil empire of all time. Jesus did not advocate rebellion or a violent overthrowing of Rome. Jesus instead exerted his efforts elsewhere. His Kingdom is not of this world, if it were, we would fight for it (Jesus to Pilate). Jesus in fact told his followers to obey the current rulers, and Paul tells us the same in Romans. This is the exact opposite mentality of so much of Christiandom's history.
Christians are called to be Peacemakers and to love our enemies, praying for the well-being and blessing of those who persecute and hate us. These crucial aspects of the Christian life cannot be accomplished through warfare or violence.
Let's input other sins into this paragraph, and see what happens.
We live in a fallen world so that means that unless the whole world turns toward God and allows Him to transform their hearts then sex and rape will happen.
We live in a fallen world so that means that unless the whole world turns toward God and allows Him to transform their hearts then injustice and starvation will happen.
We live in a fallen world so that means that unless the whole world turns toward God and allows Him to transform their hearts then teenage pregnancy and lust will happen.
All of these are true. That does not mean that they are things we are to participate in. They are all apart of this crooked world, and we belong to a different Kingdom.
There is nothing noble in losing your life, and taking other lives, so that your flag my fly over other territories, other resources, etc.
There is also no justification for the Christian in fighting against a mugger or burgular. Give to them what they ask, they are just possessions. As Daniel said, you can always get more.
And before it comes up, with the whole "What if someone was attacking your family or your self? It would be wrong not to kill them before they killed you." Please read this book.
That book is available used for less than five dollars. We can all afford that. And if you're really interested, but can't afford it, let me know, and I'll buy it for you.
And for a very simple and light 'summation' of Yoder's book, a great blog entry can be found here:The Folly of the Cross
Now, with that said, this gets to the point of the OP, now doesn't it? If we suppose these things to be true, and violence in all aspects is wrong, what do we do about it? How do we change or survive these ordeals?
The best thing you can do, yourself, is simply to live differently. To pray in earnest for your enemies, to give to those that ask, even when it leaves you cold. To provide for those in need, and to exhibit the crazy love of Christ in all interactions. It's not easy, violence can be found in more than just physical altercations. It is hard to love your enemies, and it is hard to part with things we have purchased or are attached to.
And sometimes, our commitment to the Cross leads to our deaths at the hands of evil. This is a risk we must be willing to take, when we live the life of Christ.
If you're really interested in doing more
than this, there are numerous humanitarian organizations that deal with these issues. Peace is often related to resource availability, and there are so many organizations out there providing for those who do not have. These are great ways to show, on an international level, what the message of the Cross says. There are peace organizations that work toward righting injustices, helping fix human rights violations, and live lives of example in places where there is so much violence.
I actually have a group of friends who are now living in the contested land between Israel and the Palestinians. They have chosen a dangerous life. They have joined local churches, and are attempting to be a witness to both sides of the conflict. Lives will be changed, but that doesn't mean the warring will end, because as Daniel said, until all are Christ's, that won't happen. But a small handful of people can
start a movement, and a movement, as all in America know so well, can change everything.