War is the first of the three great checks to population. It was not mainly the slaughter on the battlefield that inhibited growth, but the almost inevitable side effects. As foreign armies marched to and fro across northern Europe during the Thirty Years War, about a third of the population died, mainly from starvation and disease. Armies needed to live off the land and soldiers seized the stored grain and seed-corn, destroyed the ripening crops, killed the livestock, burned the tools.
It is also in such times that disease multiplied. With body resistance reduced by under-nourishment, and with large hordes of soldiers and camp followers coming in from outside carrying new germs, the peasants died in their thousands or sometimes millions. Epidemic diseases, in particular typhoid, cholera, plague and typhus, spread. Endemic diseases such as dysentery and malaria increased hugely. The most vulnerable, the old, women, children, will usually be the first to die, but almost everyone is vulnerable.
Tribal groups that have previously had no contact with the outside world are most at risk. Nineteen out of twenty million of the native population died when the Spanish conquered what is now Mexico. Most did not die at the end of a sword, but through famine and disease. Likewise hundreds of thousands died in North and South America and the Pacific of smallpox, influenza, measles and other diseases against which they had no immunity.
It is very doubtful whether the wars of ‘civilization’ have done anything to improve either human intelligence or physique in a selective way over the last five thousand years. They have caused horror piled on horror, a catalogue of atrocities and inhumanities which make any sensitive and informed person despair.