What is the 'lesson of humanity' (Line 11)?

Answer the question based on the following passage.

            ‘In the storm we found each other’ or ‘in the storm we clung
            together’, as Ernest L Meyer would have us say. This is
            indeed human psychology. The bond of compassion evolves
            out of shared pain. The peace movement was started by a
(5)        few academicians. They, after the First World War, came together
            to fight their common enemy - war. They discovered affinity
            despite their oblique interests and backgrounds. A sense of
            brotherhood arose out of their aim. They all pledged to fight
            for a fundamental value - ‘peace’. They joined hands to find
(10)      the strength of unity, to fight the ordeal of war. The mass
            destruction had taught them a lesson – a lesson of humanity.
            They no more could bear the strain of war. This bond of
            sympathy was a result of a bond of pain which they had
            undergone together. The storm had brought unity as
(15)      an aftermath. There seemed to be an air of confidence
            for realization of the dream to establish a perfect society
            with universal brotherhood, without bloodshed. This was the
            common desire of all the sufferers of war.