The joke is a pun.  A general commanding an army might launch a pre-emptive strike, meaning a surprise attack before the other side strikes.  'Strike' can also mean knowcking over all the pins in one roll in bowling.  Getting a strike in bowling before the names are even on the sheet could be called a 'pre-emptive strike'.
The general started bowling before his aide had entered his name on the score sheet . He had launched a pre - emptive strike .