What does the phrase "keep someone up to the mark" mean?

While I was reading George Orwell’s Letter On Why He Wrote ‘1984, I found below.

"Already history has in a sense ceased to exist, ie. there is no such thing as a history of our own times which could be universally accepted, and the exact sciences are endangered as soon as military necessity ceases to keep people up to the mark."

I wonder what does ‘keep someone up to the mark’ mean here?