Twilight of Mutual Defeat

During the Punic Wars, Rome’s martial spirit, institutional mobilization, and regenerative capacity were extremely strong. Even with 15% of its population killed in battle, it could still unite in hatred against the common enemy and, with perseverance and determination unimaginable to later generations, carry the life-or-death war to the end, never agreeing to peace.
By the time of Heraclius, Rome had already become bloated, demoralized, with sharp contradictions, and national and religious issues tearing the country apart. The Sassanid war was just the last straw. Soon it was utterly vulnerable before the Arabs.
In history, we often see two such "old empires" dragging their breathless bodies onto the ring to fight. The result is mostly a twilight of mutual defeat, never a case of one rising and the other falling. I like to call it "historical garbage time"—the Southern Song vs. Jin, the Western Jin's conquest of Wu, all were like this: a contest of who is worse, who can drag on longer.
Isn't the world today the same? The Western world, long accustomed to comfort, seems to have forgotten that just a few decades ago, the United States was able to mobilize 15 million troops and build ships like dumplings? At that time, there was a steel-like determination to resist—any bottleneck would be overcome; any sacrifice would be considered glorious; any retreat would be regarded as shame.