Well he obviously would do something in WWI but what?
He did, in a roundabout way. He was bored of the Presidency in 1908, but was enraged by taft's policies. So he went for the Republican Nomination in 1912, but Taft beat him, despite substantial popular backing for roosevelt (the bosses and old guard prefered taft). Roosevelt then formed a Progressive Party, which actually outpreformed Taft at the polls by 700,000 votes. The problem was, although roosevelt and taft between them did better then wilson, his 6,300,000 votes was enough to defeat them both seperatly.
edit: roosevelt could have won; he carried states as diverse as Pennsylvania, California, South dakota, and Washington despite taft acting as a spoiler.