Is it Scriptural for a nation to declare war?
There are three specific Bible verses that will answer this question.
In Genesis 1:28 God commands man to exercise dominion over every living thing that moves; this would obviously include other men – especially those who would harm innocent life. Following the worldwide flood of Noah’s day, God established government and gave it the right to exercise Capital punishment.
Genesis 9:5-6 says, “…Whoso sheddeth man's blood, by man shall his blood be shed…” Because God gave life, He has the authority to take life. God made government His authority on earth and gave government examples of what He sees as Capital offenses: premeditated murder (Ex 21:14); gross negligence resulting in the loss of human life (Ex 21:29); adultery (Lev 20:10; Deut 22:22); incest (Lev 20:11-12); homosexuality (Lev 20:13); bestiality (Lev 20:15-16); adolescent rebellion (Deut 21:18-21); rape (Deut 22:25); kidnapping/slavery (Deut 24:7); and several more. This authority was necessary to protect innocent life and prevent chaos.

The New Testament agrees that God has given government and local authorities to us for our own good (Romans 13:1-6). Could you imagine a month without law enforcement officials on the job?
War is just when it protects innocent life. The age of grace did not usher in changes to this divine law. The disciples were told to buy a sword, even if it meant selling their clothes to get one (Luke 22:36). Christians would be the first to acknowledge that this authority can and has been abused. Peter misused it (John 18:10-11). History has proved, however, that we as a nation have ‘justly’ entered conflicts and wars that were started by wicked men and nations. The United States must take every opportunity to protect innocents against all aggressors – foreign and domestic (to include suicide bombers).
“But,” you ask, “Aren’t we supposed to turn the other cheek?” The context of that statement is harm that comes to us because of our Christian faith. Do you honestly think that God does not want Christians to protect themselves? If a man comes into my home and would harm a member of my family, I wouldn’t offer him another member of the family…
As a nation our military strength should be used to prevent wars. But when aggressors are determined, we must be more so. I have traveled around the world. The United States of America is the greatest nation on earth. Reality TV has made it painfully clear that our justice system is greater than any other on the face of the earth. It isn’t perfect…but I’d take it over anything else I’ve ever seen!
In closing let me reiterate, God gave Capital punishment to governments. For an individual to take the law into their own hands and take human life is murder.