Defense of what?

> What does President Lessig do during the first 100 days of his Presidency if war breaks out?

He does what EVERY President does. 100 days or not (how are the # of days relevant? howabout 3! sounds a lot like political hand waving). What happens is surely that he makes a statement and the executive branch provides function. He's not a general, but he can make individual decisions just like any successor. His death or resignation doesn't cripple the country, as much as some might fantasize.

The complexity you are talking about is the same religious belief that portends that a POTUS is somehow holding the nation together. More irony, calling my characterization simplistic.