It is very, very, questionable if a Torah Jew can choose to practice Aikido over spending the same time learning Talmud. I admit wholeheartedly that of all the Martial Arts, Aikido as we have received from O'Sensei is as close as a martial art can come to be in concert with Jewish Law.
I am a (not-so) good Catholic boy and a lawyer -- so a whole heaping hopper of salt later -- on the first point, I would ask if Eccl. 3:1-8 does not state the principle on this point: "For everything there is a season, and a time for every matter under heaven: ... a time for war, and a time for peace."
The Mishneh Torah discusses the positive commandment "To imitate His good and upright ways, as it is written 'and walk in His ways.'" (Deuteronomy 28:9). Maimonides says that men are called upon to imitate every quality of the Lord, as we are made in his image. Every quality of the Lord -- both merciful and warlike. The Song of Moses (Ex.15:1-2) declares that "I will sing to the LORD, for he has triumphed gloriously; the horse and his rider he has thrown into the sea. ... The LORD is a man of war; the LORD is his name."
Some things to think about, anyway.