Charles Ellicott Commentary Isaiah 31:4

"For thus saith Jehovah unto me, As the lion and the young lion growling over his prey, if a multitude of shepherds be called forth against him, will not be dismayed at their voice, nor abase himself for the noise of them: so will Jehovah of hosts come down to fight upon mount Zion, and upon the hill thereof." — Isaiah 31:4 (ASV)

Like as the lion... — This comparison is noteworthy, not only for its fullness and vividness but also because the lion is made a symbol of protection, not destruction. Just as the king of beasts stands proudly defiant over the prey he has claimed, guarding it against the shepherds who try to take it from him, so will the LORD, in His character as the Lord of hosts, refuse to surrender Jerusalem, His special possession, to the Assyrian armies. (Compare Homer, Iliad, 18:161.)

To fight for Mount Zion. — The preposition has been translated differently as for, on, or against. In the latter case (interpreting the preposition as against), the lion is claiming the sheep as its own prey and will not tolerate outside interference. The LORD, using the Assyrian armies as His instruments, will fight against Jerusalem and will not allow the Egyptian allies to interfere with His punishments. (Compare Isaiah 29:7-8.) The second rendering, on, simply marks Jerusalem as the scene of the conflict, but agrees in substance with the first, for. Considering the following verse, the idea of protection seems more natural than hostility. The thought of supreme ownership, however, includes both aspects: Jerusalem belonged to the LORD to protect or to punish.