Does a Globe of Invulnerability protect against the Magic Weapon spell?

Question

Consider a scenario where a party of adventurers encounter a lich. The party is armed with mundane weapons which can't hurt the lich so the spellcasters begin casting magic weapon. The lich casts globe of invulnerability in response to this and stays inside of it. So what happens when the party starts attacking the lich? According to the globe of invulnerability spell description:

Any spell of 5th level or lower cast from outside the barrier can't affect creatures or objects within it, even if the spell is cast using a higher level spell slot. Such a spell can target creatures and objects within the barrier, but the spell has no effect on them. Similarly, the area within the barrier is excluded from the areas affected by such spells.

Is magic weapon excluded from working inside the globe because of the last sentence?

Answer 1

Magic weapon will work against the lich

Regarding the last sentence specifically, magic weapon does not affect an area, it only affects a target (an object). Thus, this sentence has no relevance to this spell. The only purpose of this sentence is to exclude the globe from area of effect spells like fireball and the like.

Magic weapon affects one target: a nonmagical weapon that you touch. That weapon then becomes a +1 magic weapon for the duration. When you then try to use the weapon against the lich you are not affecting the lich with the spell, you are affecting them with the newly enhanced weapon. Thus, nothing about doing this would be blocked by globe of invulnerability. The same is true if you cast enlarge or invisibility or polymorph on yourself and moved into the globe to attack.

If globe of invulnerability did work against magic weapon it would lead to ridiculously powerful results such as creatures under the invisibility suddenly become visible since the spell "affects" the lich by way of making the PC impossible for them to see. Or that mirror image stops working because it "affects" the lich by making it hard for them to hit the PC. In essence it would make this spell on par with antimagic field, an 8th level spell.

Answer 2

Casting order matters

My opinion on this changed after considering "Can I stop previously cast spells from affecting me by moving into a globe of invulnerability?"

In your example, the lich cast globe of invulnerability after the party cast magic weapon. The globe only blocks spells cast from outside. Before globe of invulnerability is cast, there is no inside or outside of the globe, because there is no globe. Therefore, it does not affect spells that were cast before it.

Magic weapon will not work within the globe if cast after it

If the lich instead cast globe of invulnerability before the party cast magic weapon, the answer is different.

The spell magic weapon is cast on a weapon, not on the lich itself, so does it still count as affecting the lich? For this spell in particular, it doesn't matter. The weapon is an object and globe of invulnerability prevents objects from being affected by spells cast outside the globe. It doesn't matter if the object was previously outside; once it is used to attack, it (or its ammunition) must enter the globe, and thus cannot be affected by magic weapon.

Word of God

For more about what it means for a spell to affect a creature, we take a look at the Rakshasa (MM 257), which has the following property:

Limited Magic Immunity. The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be.

Jeremy Crawford has answered a question about this property in a Sage Advice tweet.

@JeremyECrawford Hi Jeremy, does a Rakshasa's 'Limited Magic Immunity' protect it from weapons enhanced by shillelagh or the magic weapon spell since they are spell effects?

- Vitrax (@Spooky_Wizard)

Shillelagh and magic weapon are spells. Limited Magic Immunity prevents a rakshasa from being affected by spells of 6th level or lower, unless it wishes to be. #DnD

- Jeremy Crawford (@JeremyECrawford)

It seems clear that "the rakshasa can't be affected" is equivalent to "any spell [...] can't affect". Thus, magic weapon will not affect the lich if cast after the globe.