From The General Theory Chapter 24, section IV:
War has several causes. Dictators and others such, to
whom war offers, in expectation at least, a pleasurable excitement, find
it easy to work on the natural bellicosity of their peoples. But, over
and above this, facilitating their task of fanning the popular flame,
are the economic causes of war, namely, the pressure of population and
the competitive struggle for markets...
[U]nder the system of
domestic laissez-faire and an international gold standard such as was
orthodox in the latter half of the nineteenth century, there was no
means open to a government whereby to mitigate economic distress at home
except through the competitive struggle for markets...
But if
nations can learn to provide themselves with full employment by their
domestic policy (and, we must add, if they can also attain equilibrium
in the trend of their population), there need be no important economic
forces calculated to set the interest of one country against that of its
neighbours. There would still be room for the international division of
labour and for international lending in appropriate conditions. But
there would no longer be a pressing motive why one country need force
its wares on another or repulse the offerings of its neighbour, not
because this was necessary to enable it to pay for what it wished to
purchase, but with the express object of upsetting the equilibrium of
payments so as to develop a balance of trade in its own favour.
International trade would cease to be what it is, namely, a desperate
expedient to maintain employment at home by forcing sales on foreign
markets and restricting purchases, which, if successful, will merely
shift the problem of unemployment to the neighbour which is worsted in
the struggle, but a willing and unimpeded exchange of goods and services
in conditions of mutual advantage.
Peace.