The Lord will be their protection, as he was with Gideon and his three hundred who slaughtered the Midianites. The Lord informs Jerusalem that their salvation will be miraculous, analogous to the miracle at Midian, and the miracle of their deliverance from Egypt when they walked on dry land through the sea.
Historical information: "In verse 12, Isaiah gives the first hint that Jerusalem will actually be attacked by Assyria as was Samaria. The Assyrian invasion no doubt fulfilled a purpose the Lord had in mind; he wanted to humble the Jews and awaken them to an awareness of him. The invasion referred to took place under Sennacherib in the year 701 B.C. During this invasion, many cities in Judah were destroyed, but Jerusalem miraculously held out against the Assyrian siege because the Lord sent a terrible sickness throughout the Assyrian camps, which caused many deaths. (2 Kgs. 19:35, 37; Isa. 36-37.) (Ludlow, 1982, p. 163.)