He had opposed British involvement in the war at its outbreak in 1914, a highly
principled position to take in the face of its huge popularity among members of his own party,
resigned his position as the leader of the Parliamentary Labour Party in consequence and then had his illegitimate birth raised
by the newspapers in an attempt to get him to
resign his seat as an MP (they argued that as he'd used a different name during his life from that on his birth certificate he'd stood for election on false pretenses and deceived his constituents).