What does the poet mean when he says “by war’s long winter starv’d”?

Explanation

If a war is raging in a country, then that country faces the threat of starvation since all agricultural production comes to a halt. Just as there are no crops in winter, war renders a land barren. That is why there is a shortage of food in winters and in times of war, too, there is deprivation and famine. People starve to death. Thus, starvation is associated with war and with winter.