The Marauders didn't want to learn all the Dark Arts. Just the spells and their counters they might encounter in a fight with Death Eaters. The theory behind the spells didn't interest them. Nor did the more esoteric of the available spells. Only those they'd most likely encounter when they crossed wands with the Death Eaters. Thanks to Lily, they'd realized the simple light spells they were taught at Hogwarts wouldn't serve them well in a war where the other side was using dark spells they had no knowledge of and couldn't shield against or properly counter. As Lily had told James, "If you truly want to avenge your parents, wouldn't it be better to survive your first encounter with the group that killed them so you can get them all before they escape justice and go on to kill other peoples parents?"
So the Marauders, much to Black's disgust, had sat down with Severus to learn real Dark Arts instead of just light spell defense against the unknown spells of a Dark Arts Master. Lupin never came to very many of their study sessions and tried very hard to get the other Marauders to renege on them as well. But Lily wasn't to be swayed and where she went, James followed. Where James went Black followed.
When Lupin did come, he made it very clear he wasn't comfortable learning the Dark material Severus was trying to teach them. He tried very hard to make Severus only show them the light spell counters to the different dark spells without actually demonstrating or explaining anything at all about the dark spells themselves. He claimed just hearing about the dark magic spells brought his inner wolf too close to the surface and was making it harder for him to control the beast. Severus still didn't know if that was true or not. He rather thought it wasn't. Somehow he rather thought it was more because Albus said it wasn't necessary to learn Dark Spells to defeat them. Much like Granger in the here and now, Lupin listened to and trusted the Headmaster far too much for Severus' comfort.
But then Albus truly believed the light side shielding spells he had the Hogwarts Professors teach the student base could defend them against any Dark Magic spell cast at them. The problem with that was certain shields only worked against specific spells and others had no counter shield at all. If you didn't know which spell was being cast in your direction, which many light side fighters didn't even when they heard the spell being cast at them, it'd be just as likely you'd cast the wrong shield spell as not. Or fail to summon something solid into the spell's path in time to save your own life.
During the war Lily and the Marauders had saved many a life because of their seventh year study sessions. They'd hear a spell being cast, see who was targeted by it and cast the appropriate shield for the intended victim. Or summon something into the oncoming path of the destructive spell.
Even back then, in their seventh year, Lily and the Marauders had all known in war there wasn't any such thing as getting back on your feet and having a do-over so you could cast the right shield the next time. Death Eaters played for keeps. The daily storm of black letter carrying owls in the Great Hall was telling them all that simple truth. Even when they'd been seventh years, it was clear the Ministry aurors were fighting a losing battle against the tide of the Dark Lord's forces.
Still, he truly wondered sometimes if it wasn't their knowledge of the Dark Arts that had led to their early deaths though he had no way to find out. Their competence in battle would've told Albus they had a greater knowledge of the Dark Arts than he'd have been comfortable with. Albus swore, and Minerva believed, that all dark magic corrupted the user no matter what the purpose was in learning it. Neither one would believe for even the blink of an eye, that Light magic couldn't meet and excel over Dark without the light magic fighter learning a single thing about the dark magic they were opposing. Lily and the Marauders would've been an embarrassment to Albus simply because he'd know it was their knowledge of the dark arts spellwork that allowed them to battle and triumph over the dark forces so often.
Any time someone tried to point out how luck had played a large role in any victory the light side had, Albus would smile, twinkle his eyes and point out how very long he had lived as proof the Dark couldn't truly triumph over the Light. He never actually denied their victories were more due to luck than skill. Luck, that someone at the site of the battle had been more knowledgeable of the tactics being used and had the ability to counter them. Usually Lily and the Marauders. And sometimes himself.
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