Adam Clarke on Judges 12:3

I put my life in my hands - I exposed myself to the greatest difficulties and dangers. But whence did this form of speech arise? Probably from a man's laying hold of his sword, spear, or bow. "This is the defender of my life; on this, and my proper use of it, my life depends." When a man draws his sword against his foe, his enemy will naturally aim at his life; and his sword in his hand is his sole defense. It is then, Fight and conquer, or die. Thus Jephthah took his life in his hand. This phrase occurs in some other places of Scripture; see 1Sam 19:5; 1Sam 28:21. And the words of the Conqueror, Isa 63:5, seem to confirm the above view of the subject: I looked, and there was none to help; and I wondered there was none to uphold; therefore mine own arm brought salvation unto me; i.e., by mine own arm I saved my life, and brought destruction on mine enemies.