What connection did Phineas make between jumping from the tree and the war effort?

The older boys trained for the war effort at the same site, and the same tree. Thus, Finny saw his act of jumping from the tree as equal to their own. None-the-less, when he states, "here's my contribution to the war effort!", Finny is joking. He and the other boys have not yet been affected by the war.... it hasn't goteen that close.