Or HaChaim on Exodus 15:3/אור החיים על שמות ט״ו:ג׳

1 ה׳ איש מלחמה, "The Lord is a man of war," etc. This means that even when G'd operates with the attribute of Mercy He is still "a man of war." The fact that He conducts war does not imply that He abandons the attribute of Mercy. His name remains י־ה־ו־ה and all that this implies just as we find in Maleachi 3,6: כי אני ה׳ לא שניתי, "for I the merciful G'd have not changed, etc." G'd has made this plain also at the beginning of פרשת וארא where He told Moses אני השם, I am the attribute of Mercy.