They did not hang the screen door properly. :-)
Prior to World War I there was a worldwide build up of capital warships, which went hand in hand with mutual defense treaties. So when there was an outbreak of war, suddenly many nations were involved.
Today there is a worldwide build up of submarines. 46 nations now have them, and while there are few nuclear submarines, there are increasing numbers of very quiet, air independent diesel electric submarines prowling the oceans of the world.
It is a good question whether this will result in war, overt or covert, with some suggestions that submarines have already engaged in belligerent attacks against surface ships carrying nuclear proliferation cargoes.
However, an even better question is whether one or more of the major powers will decide that too many submarines at sea represent an inherent threat, and to covertly start culling their numbers. With plausible deniability, of course.