Poor Pyun. Poor Sollies.
So where was BuShips, Missile Defense Branch Desk in the Solarian League's Navy during the last what, fifteen years of Honorverse time?
Busy developing ECM drone networks and countermissile canisters to be fired out of main missile tubes. Solarian missile defense is actually really good
by the standards of, well, anything deployed prior to 1914 Post-Diaspora. If it weren't for the multiple-drive missile I suspect they'd actually be stopping most of what was thrown at them.
The problem is that the transition from single to multiple drive missiles is a lot like the transition from piston engine aircraft to jets. It's not just that it makes your defense problem harder, it's that you have to back up and rethink from first principles how to defend.
It makes entire categories of defensive systems obsolete (e.g. AA guns become nearly useless and give way to guided missiles).
But whereas Manticore was preparing to defend against MDMs probably as early as 1905-1910 (because they knew that was coming), and Haven has been engaged in maximum-effort research to defend against them since 1915 (when they started having MDMs shot at them), the Solarian League remained in collective denial up to about 1920. Note that all use of MDMs prior to 1921 took place in star systems not
immediately adjacent to Solarian space (i.e. not in Manticore or anywhere else you'd expect massive Solarian presence as a matter of routine).
We have indications that private corporations within Solarian space are
engaged in R&D that would lead them to viable defenses against MDMs... but that hardware simply has not reached the Solarian Navy yet.
So this is basically Imperial Japan suddenly developing an electronics industry many times more advanced than that of the United States circa 1945.
No, it's more like South Korea fielding more advanced and capable weapon systems than, say, China. Or the USSR, if it still existed.
Manticore has been obsessively reinvesting the huge piles of free money it gets from its big friendly spatial anomaly in advanced weapons technology for about sixty years at this point. They're a lot closer to the edge of the technological envelope permitted by 'modern' technology than the League is, simply because they have an active R&D cycle
. Much more active than that of the League.
As a result, they've been able to pull ahead on the military R&D curve to build a lead of about one decade, while working hard for the past six
decades, with the League not really even trying or even being aware it was in a race.