The Schlieffen Plan had to be altered before even a shot was fired, because the manpower required was more than Germany could mobilize and train without tipping off everyone. Not long ago I read about the finding of documentation that the plan had actually had a formal existence — apparently there was a school of alleged thought that said that there never was such a plan. Imagine!
The Germans (and the Austrians) achieved battlefield supremacy through superior firepower (along with better training, better leadership, better command structure, and a coherent plan). When they were under attack by massively superior numbers, they merely staged orderly retreats, let the allies “pay for the same real estate twice” (line from the movie “Patton” I’ve often wanted to use), let the allies have undefendable ground and force them to assault easily defendable spots, let them punch themselves out, then shifted resources, and wiped the battlefield clear of them. The BEF had wrecked itself by the time the US entered the war, and until 1917, the German casualty figures never approached those of the allies.
The French actually figured out the problem after a couple of years (the British never did, and apparently keep peddling their wartime propaganda in their schools and histories of the war), and sent people from their most successful units to assist in training the US army. Thanks, France. Of course, they made up for it by selling us really lousy guns, but we shoulda/coulda used the Browning. ;’)
I've heard of a school of thought who believe or believed that Schlieffen never intended his plan to be used, that he meant it as some kind of staff exercise.
It was truly tragic how many lives the British lost in the repeated battles for "wipers."
The more I reflect the more important I think WWI is in our history. Everything changed afterword and most of the trends we see today began as a consequence of the awfulness of that war, distrust of nationalism, distrust of the military, multilateralism, pacifism, etc. Only the clearly perceived need to defeat Hitler's evil was enough to delay these trends for a time. I just read the first volume of Churchill's WWII memoirs and it's amazing how much of the nuttiness that drives we conservatives to distraction in foreign and military affairs was already evident during the interwar years.