"Learning in Wartime" is about just that, learning. This was written at the time of World War II. Lewis is telling his audience that just because there is a war going on at present, that does not mean that learning should stop. For, if we wait for a time when there is nothing to worry about or for life to become "normal" again, we will never get to learning anything. According to Lewis, war does not make life not norma; it only amplifies the difficulties we already struggle with. Learning is an ongoing process that cannot simply come to an end only because larger difficulties come our way.
Lewis talks about three enemies that war brings. The first is excitement. In war time it is difficult to think about anything besides war, especially when you are affected drastically. This overexcitement about the war makes it very difficult to think about anything regarding work or learning. This cannot be if we want to fulfill God's calling for us in our lives.
The second enemy Lewis talks about is frustration. Everyone becomes frustrated with not being able to finish everything. The frustration is that the war will bring an abrupt end to learning and leave us unfulfilled. Lewis argues this by saying that no matter when our end comes, we will always have something else that we could have done. No one has finished everything that they wanted to get done before they died. So, the fact that war is going on should not add to the frustration of not getting anything done since you will not get it all done anyway.
The last enemy is fear. The fear is that you will suffer death. Lewis puts an abrupt end to this fear by saying that war does not increase chances of death. The chance could not be increased anyway because there is already a 100 percent chance of death. There have only been two exceptions to this rule in all of history. Therefor, death should not be feared any more in wartime than in "normal" times.