Justin Bieber to Orlando Bloom -- I Balled, Now You Bawl

Justin Bieber has declared Instagram war on Orlando Bloom ... with a pic of the Brit dissolving into tears.

It's Bieber's second Instagram salvo ... hours after Orlando took a swing at him at Cipriani restaurant in Ibiza he posted a sexy pic of Miranda Kerr and then quickly took it down.

Miranda -- the Helen of Troy in this war -- was the trigger for the bar scuffle. It's clear Orlando felt Justin was banging her while they were still man and wife.

We'd say there's no use crying over Miranda, but then again ...