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In the Harry Potter universe disarming your opponent seems to be the most logical opening move in a duel, but only Harry seems to do this often. Is there any in universe explanation for this, for example an Expelliarmus counter spell?

I can understand why you might go for Avada Kedavra if you wanted to kill someone as quickly as possible and were prepared to use it, but many duels seem to last for several spell exchanges that could be avoided if you just disarmed your opponent.

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    A proper duelist believes in the heart of the cards, er, wand. – user16696 Feb 26 '15 at 18:12
  • That said, aren't low level spells like that easily ignored? – user16696 Feb 26 '15 at 18:12
  • You mean shielded from? Lupin gets concerned about it becoming Harry's signature spell, are there any examples where its failed where a 'stronger' spell hasn't? – Pont Feb 26 '15 at 18:19
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    Related (and maybe a dupe answer buried in there): What are the rules for magical duelling? – phantom42 Feb 26 '15 at 18:21
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    @cde I suppose that raises the question of why Harry isn't dead then, considering he mostly stuns and disarms. Maybe it's just the +5 plot armour :P – Pont Feb 26 '15 at 18:29
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Expelliarmus is a basic, low level, primitive spell. Well known, easily countered (shield, block). It's also not harmful, so if it misses/is blocked, you wasted "a turn", so to speak, allowing your opponent to attack unmolested.

As the old saw goes, the best defense is a good offense. Starting a duel with an offensive spell seems to make a lot more sense - if it connects, you progressed in the duel; if it doesn't, the opponent was busy countering it.

Imagine this game theory scenario breakdown:

"Counter" means shield or dodge/block "Exp.OK" means Expelliarmus of yours that succeeded "Exp.FAIL" means Expelliarmus of yours that failed.

Note that due to Expelliarmus's basic nature, the chances of it succeeding is always HIGH on both sides, unless the opponent blocks (then chances are low instead, again due to basic nature)

You:      Opponent            Result               Probability  Advantage
============================================================================
Exp.FAIL  Counter (success)   Status Quo           High         None
Exp.OK    Counter (failed)    Opponent disarmed    Low          You
============================================================================
Exp.OK    Exp.OK              Both disarmed        High         None
Exp.OK    Exp.FAIL            Opponent disarmed    Low          You
Exp.FAIL  Exp.OK              You disarmed         Low          Opponent
Exp.FAIL  Exp.FAIL            Status Quo           Low          None
============================================================================
Exp.FAIL  Weak Offensive      You suffer effects   Low          Opponent
Exp.OK    Weak Offensive      Opponent disarmed+   High         None
                              You suffer effects                
============================================================================
Exp.FAIL  Strong Offensive    You hurt/dead.       Low          Opponent
Exp.OK    Strong Offensive    Opponent disarmed+   High         Opponent
                              you hurt/dead.                    
============================================================================

So....

If you open with Expelliarmus:

  • If the opponent opens with a block/shield/dodge, most likely, it's a wash. No advantage to anyone
  • If the opponent opens with Expelliarmus, most likely, it's a wash. No advantage to anyone.
    • There's a small chance that you disarm them and they fail to disarm you. Due to the basic nature of the curse, that's unlikely.
  • If the opponent opens with a weak curse, they are most likely disarmed but you are hurt. This is a wash - you can recover from a weak curse; and they can retrieve their wand.
  • If the opponent opens with a strong/lethal curse, they are most likely disarmed but you are severely disabled/dead. Fat lot of good it does you to have them disarmed in exchange.

So...

  • in a real duel (where the goal is to take out the opponent), the BEST strategy to counter Expelliarmus is to fire a strong offensive curse, and it's a stably winning strategy. Thus, Expelliarmus is a very bad opening.

  • In non-real school duel, most likely outcomes are ones with no-win or opponent win. The only outcomes with your advantage are those that have very low probability (opponent shields but shield fails; and opponent casts Expelliarmus which fails). So, Expelliarmus isn't really worth it, it's a wash at best as an opening. And the school duels are mainly training for real duels so aquiring bad habits from early on is a Bad Idea.

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  • Great Answer, very detailed and well constructed. Although, there's also the change that the Weak Offensive spell may not be strong enough to prevent the opponent from following up on his own attack (eg Expelliarmus), or did you count that as in Failed? – Oak Feb 27 '15 at 2:17
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    You have a typo- you say Exp.FAIL == succeed, shouldn't it be Exp.FAIL == fail? Since you say Exp.OK == succeed ? – LepelLeLama Feb 27 '15 at 6:49
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    @LepelLeLama - good catch! If you make the edit, you will earn 2 reputation points and my gratitude :) – DVK-on-Ahch-To Feb 27 '15 at 14:33
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The only canon answer I can think of, is that Expelliarmus is blockable. Dark curses, such as Avada Kedavra, either have no block, or like Sectumsempra, have no cure* (George's ear).

*Having no cure meaning that a body part that is detached using it, it can't be repaired/regrown. While being struck with the curse, does actually have a cure(Snape healed Draco).

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Well if your intent is to kill, you might as well open with AK, if you can cast it just as easily and quickly. If it hits you're done with, and can move on to another target (in context of a battle or a fray).

When dueling for sport, it might be a very good opening move, but it might be also quite predictable - specially if you overuse it. We can presume, that spell can be more easily defended, if you know it's coming. So it might make sense to open with something unpredictable, and get a little creative with your spell choice. I guess part of why Harry was so successful with Expelliarmus was that noone expects it to be used in life-or-death situation, when death curses are flying left and right.

Also dueling might be a bit like chess, if you open the same every time you will be easily countered, and although a Fool's Mate, might be the fastest way to win, no formidable opponent will fall for it.

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  • Avada Kedavra is an Unforgivable Curse, the opening section of the wiki article says they were made legal for Aurors to use in battle during the first two Wizarding Wars, but in general I think the good guys aren't allowed to use them even in life-threatening situations. – Hypnosifl Jun 16 '16 at 23:54
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If the wizard does Expelliarmus as an opening move, the oppenent can do Protego. So, disarming isn't always useful whilst duelling with professionals like Harry or Malfoy.

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