Actually I want to split the hair of what's "senseless" and "terrible failure."
Gallipoli--a "terrible failure"--was not "senseless" before it began. Given what they knew, the Brits were confident of success, but they clearly didn't know enough.
Same for Gettysburg--If you were Bobby Lee that morning, did you really think failure was the highest likely outcome?
On the other hand, I don't see the justification the same Brits felt prior to the Somme. That even after Verdun they believed their bombardment would leave the enemy either as paste or gibbering lunatics was based on no evidence that their success was guaranteed. If virtually 60,000 casualties in a day isn't "senseless slaughter," I can't imagine what is.
Agincourt probably qualifies for "SS" but only because the French Knighthood never seemed to approach battle with any other plan than "Banzai!" (or the contemporary socio-economic equivalent of Middle Ages French). The Brits had no choice in the matter but to fight--but they knew how to fight that enemy.
Similarly, Stalingrad, Kursk, and Ardennes Offensive were all idiotic ideas from the first, and none had any real chance of success. All coming from the Fuehrer's "Black Hole" of a heart/brain, they couldn't have been anything but
In short, where there really seemed to be a reasonable chance of success, the "SS" was an after-the-fact appreciation of the outcome. But where there was no real chance of success, but they did it anyway, then an "SS" rating is spot on.