Is there any biblical support for any country or human being to be in all-out war or conflict?
Actually there are hundreds of passages that support the concept of war. War is one of those topics that pacifists have to work hard to try to get it out of the Scripture. To keep this relatively short, I'll just point out a few.
God commands Israel to go to war many times. On occasion, he has ordered the complete destruction of a nation (what we would term as genocide in our day). On one such occasion, he not only ordered the complete annihilation of nation, but he punished the king of Israel for leaving one man and a few sheep alive. King Saul was told in I Samuel 15:3 "Now go, attack the Amalekites and totally destroy everything that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys." By verse 8 we are told, "He took Agag king of the Amalekites alive, and all his people he totally destroyed with the sword." In verse 10 and 11 we learn "Then the word of the LORD came to Samuel: 'I am grieved that I have made Saul king, because he has turned away from me and has not carried out my instructions.' Samuel was troubled, and he cried out to the LORD all that night." A few verses later we learn that God rejects Saul as king.
In the New Testament, we find some different kinds of passages. God does not deal with his people on a national level like he did in the Old Testament and his people are now spiritual people. However, the spiritual people of the New Testament have to live in the reality of the world as it exists -- which includes armies and war. So, at no time are we told to take up arms to advance the kingdom. We are told what our attitude toward the military and war should be.
"Then some soldiers asked him, "And what should we do?" He replied, "Don't extort money and don't accuse people falsely—be content with your pay"" (Luke 3:14).
In this passage, John the Baptist was asked by soldiers on how to repent. I think it is significant that John did not say anything like "give up being soldiers". He told them to do some things that soldiers of his time were known to have problems with. I find John's reply very telling because John was not afraid to "tell it like it is" -- since he eventually lost his head for telling the king that he was guilty of adultery. If being a soldier was being immoral, John would have said something.
"At Caesarea there was a man named Cornelius, a centurion in what was known as the Italian Regiment. He and all his family were devout and God-fearing; he gave generously to those in need and prayed to God regularly. One day at about three in the afternoon he had a vision. He distinctly saw an angel of God, who came to him and said, "Cornelius!" Cornelius stared at him in fear. "What is it, Lord?" he asked. The angel answered, "Your prayers and gifts to the poor have come up as a memorial offering before God" (Acts 10:1-5).
Cornelius ends up being the first Gentile convert to Christ. His status as a Gentile caused problems for Peter, so there is a lot to the story about how this man was chosen. Again it is telling that Cornelius was also a soldier -- and considered so worthy that he was chosen to be the first Gentile convert. At no time is he told to quit being a soldier in order to become a Christian, nor is there any record of him changing occupations. He is a Christian and a soldier.
"Everyone must submit himself to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God. Consequently, he who rebels against the authority is rebelling against what God has instituted, and those who do so will bring judgment on themselves. For rulers hold no terror for those who do right, but for those who do wrong. Do you want to be free from fear of the one in authority? Then do what is right and he will commend you. For he is God's servant to do you good. But if you do wrong, be afraid, for he does not bear the sword for nothing. He is God's servant, an agent of wrath to bring punishment on the wrongdoer. Therefore, it is necessary to submit to the authorities, not only because of possible punishment but also because of conscience. This is also why you pay taxes, for the authorities are God's servants, who give their full time to governing. Give everyone what you owe him: If you owe taxes, pay taxes; if revenue, then revenue; if respect, then respect; if honor, then honor" (Romans 13:1-7).
This one is a little more complex. It is obvious that Paul is saying that governments exist at the will of God. We like to think that they exist because of the will of man, but Paul said they actually work for God. God has granted to them the right of using the "sword". A sword does not have many uses outside of police work and military might. Paul is saying that we have to submit to the authorities because they are established by God as his agent to execute his wrath. The execution of wrath is not limited to police work. Sometimes a whole nation becomes so evil that the only recourse left is for God to punish the whole nation. The only difference between police work and war is a matter of size. In our society, we also change the rules, but the effect is the same.