Which war? however in general, a high unemployment rate after a war is because of the high amount of solders coming home and looking for new jobs after they are not needed in the military anymore, the economy usually can't support this so there is often a period of high unemployment as people slowly filter into older jobs. Another reason, especially in more modern wars, is that some jobs are not needed anymore, in the World Wars, our economy became rather militaristic, however, after the wars ended we no longer needed no produce so many weapons so people are laid off. A third reason for the more modern wars is that women started to replace men in the workforce, and because they were paid less business owners did not want to go back to men, who they had to pay more.
Hope this helps!