The first reference to war in the OT occurs not long after the call of Abraham.
1 In those days Amraphel king of Shinar, Arioch king of Ellasar, Chedorlaomer king of Elam, and Tidal king of Goiim 2 waged war against Bera king of Sodom, Birsha king of Gomorrah, Shinab king of Admah, and Shemeber king of Zeboiim, as well as the king of Bela (that is, Zoar). (Gen 14:1-2)
The alliance of kings wins, and they pillage Sodom and Gomorrah . Lot, Abraham’s nephew who was living in Sodom, was taken captive.
14 When Abram heard that his relative had been taken prisoner, he assembled his 318 trained men, born in his household, and they went in pursuit as far as Dan. 15 And he and his servants deployed against them by night, attacked them, and pursued them as far as Hobah to the north of Damascus. 16 He brought back all the goods and also his relative Lot and his goods, as well as the women and the other people. (Gen 14:14-16)
This is the point that the mysterious Melchizedek, priest-king of Salem, appears:
19 He blessed him and said: Abram is blessed by God Most High, Creator of heaven and earth, 20 and give praise to God Most High who has handed over your enemies to you. And Abram gave him a tenth of everything. (Gen 14:19-20)
Much has been made (and can be made) of the theological significance of Melchizedek. However, it is probably safe to conclude that he was speaking for God in this occasion. This blessing shows us the beginning of a theology that God can and will act for the good of his people on the international stage.While there is no mention of the size of the alliance’s army, it is likely that a war of the scale described involved an army of more than 318 men. Abraham’s victory was given to him by God, not his own might. Abraham’s offering of a tenth suggests his own recognition of God’s actions, and his appropriate response.