Two things should always be remembered,
Christianity can survive under extremely hostile environments. It was born in the hostile world of the Roman Empire. In our own time, Christianity has blossomed under the hostility of both the Soviet Union and Communist China. Therefore, Christianity will never be dependent on winning a war for its survival. If a Christian government has no hope of defending against attack, it should surrender, knowing that Christianity will survive.
It is God who determines the appointed times of the nations and the boundaries of there habitations. (Acts 17:26, cf Job 12:23, Deut 32:8) If a nation is invaded by another and this is not God's will, he will not allow the situation to last long. For example, after the Second World War, the Russian Empire took control of most of Eastern Europe. However, because this was contrary to God's will, that empire had collapsed within fifty years. If a nation is unable to defend itself, all is not lost; God will have his way in the end.