There have, of course, been many changes in warfare over the past 250 years. The most obvious changes have come about through technology. In 1763, people were fighting with single-shot, muzzle loading long arms and artillery. Today, we have machine guns and tanks and aircraft. These things have made war tremendously different on a tactical level than it was 250 years ago.
However, perhaps the greater change is that major wars in modern times are now total wars. This was a concept that did not truly exist 250 years ago. Because of technology, major wars today are generally won or lost based on the technological and industrial capacity of the combatants or their supporters. These wars (which are rarer than they were 250 years ago) are typically won by whoever has the most and the best weapons and supplies. Because of this, whole societies are part of wars. War is no longer just about armies but about industry. It was for this reason that (for example) so much of WWII focused on efforts to destroy the enemy’s industrial centers and supply lines.
Thus, we can argue that the biggest change in the past 250 years is that war today is more of an industrial and technological contest than it was before.