Article: Letters of support from the pope arrived in April but by then the rebel barons had organised. They congregated at Northampton in May and renounced their feudal ties to John, appointing Robert fitz Walter as their military leader. This self-proclaimed "Army of God" marched on London, taking the capital as well as Lincoln and Exeter. John's efforts to appear moderate and conciliatory had been largely successful, but once the rebels held London they attracted a fresh wave of defectors from John's royalist faction. John instructed Langton to organise peace talks with the rebel barons.

Now answer this question: Who was the rebel baron leader?
Robert fitz Walter