Missile defense systems constitute first-strike technology.
I think it is dual-use. A strong (compared to the offensive capabilities of a possible adversary) missile defense is a first-strike technology. A weak missile defense can be a very important second-strike technology.
E.g. in the context of US-China: Given the differences in the amount of warheads and delivery systems a strong US missile-defense system could be a destabilizing first-strike technology. On the other hand, a weak Chinese missile-defense could be stabilizing by enhancing the second-strike capabilities of China without harming the second-strike capabilities of the US.
I think it is dual-use. A strong (compared to the offensive capabilities of a possible adversary) missile defense is a first-strike technology. A weak missile defense can be a very important second-strike technology.
E.g. in the context of US-China: Given the differences in the amount of warheads and delivery systems a strong US missile-defense system could be a destabilizing first-strike technology. On the other hand, a weak Chinese missile-defense could be stabilizing by enhancing the second-strike capabilities of China without harming the second-strike capabilities of the US.
You’re not wrong. Context does indeed matter. Few systems fall perfectly into first-strike vs second-strike.