War always is waste, Jim, but it's never pointless if you're being attacked. The problem is to identify an attack. It's easy to do that if your own country is being invaded, but consider the situation in which the United States found itself prior to WW II:
Most of our people wanted to stay out of that "European" war. But if we'd gotten into it early and the Western democracies had brought Hitler to heel while he was relatively vulnerable, the fight would have been much smaller. As it happened, we had to wait until we were attacked in Hawaii to join the fight, and by then, instead of the forces of a single dictator on a single front we had to face the forces of three on multiple fronts. By being unwilling to face the waste of a relatively small war we had to face a worldwide catastrophe.
And of course Britain wasn't any smarter. Churchill pled with the British and the French to smack down Hitler before he could build his forces to a dangerous level, but nobody wanted to listen. As he put it in those days, "The Hun is always either at your throat or at your feet," and history clearly bore him out.
Then we watched the Soviet Union begin to swallow as much of Europe as it could, and communists in the East begin to encroach upon their neighbors. If the West hadn't shown its willingness to fight in places like Korea and Vietnam -- especially Korea -- do you really think those cancers wouldn't have expanded and metastasized?
Now we're right back where we were in the late 1930's, this time watching the rise of Islamists who want to take over the world and who now, because nuclear weapons exist, can create catastrophe without raising a large military force. And yet the general reaction among the Western democracies seems to be: "Ho hum. No big deal." We even have politicians trying their best to downplay what's an existential threat.
Your granddad was right, in a sense, but you also always have to look beyond the point "where ignorant armies clash by night," and understand what evil forces have brought on the clash. Sometimes, as in the late 1930's, the future's not awfully hard to predict, and what needs to be done isn't hard to see. Sometimes that's war.