Continuous year-round war is a modern invention. Up through the 19th century, war in temperate zones started in the spring and ended when the cold winter rains or snow arrived; in other climates, war was waged between monsoons or hot seasons. When the weather turned, both sides retreated to winter quarters, where they rested and planned for the next battle season. (Of course, this meant that a winter attack was a sure way to surprise your enemy, if you could pull it off.)
The American Revolutionary War started on April 19, 1775, and was fought vigorously spring through fall; the most famous winter quarters for Washington's troops was Valley Forge, where they spent the winter of 1777-78 before heading out to fight the British again in late June, at Yorktown. In 1812, Napoleon's campaign against Russia, which started in June and saw French troops in Moscow by fall, was eventually stymied by the rigors of the Russian winter; by the time Napoleon's troops returned to France, the brutal early winter weather had killed most of the men who had survived the battle for Moscow. Although the South seceded in January of 1861, the American Civil War began in the spring, on April 12, 1861; each year, fighting continued until December or January, then the troops went into winter quarters until the worst of the cold and the mud were over, in April or May. The one major exception to this was Sherman's march through the Carolinas, which marked the beginning of the end of the war.
It wasn't until the 20th century that wars were routinely fought through the winter. It is a measure of how modern technology cushions us from the weather that fighting seasons no longer seem obvious.