> How can it be too far to install an anti-missile system on your own territory? That seems eminently reasonable and uncontroversial to me. It's not even a euphemism is it? It literally is purely a defensive system?
This is an old argument that goes back to the cold war. ABM systems were prohibited by treaty because an effective ABM system is not purely a defensive system. It breaks the deadlock described by Mutually-Assured Destruction. MAD means neither side can rationally initiate a first strike because an attack will result in the destruction of both sides. An ABM system that actually works, controlled by only one side, means that side can make a first strike and not be destroyed.
That is not necessarily an argument against developing defensive systems. During the eighties one thing people suggested for addressing this was to essentially give US-developed ABM technology to the USSR, or to jointly develop the technology, etc. The system's deployment would be followed by a gradual reduction in nuclear stockpiles by both sides. The thinking being, if both sides possess the defensive technology the amount of tension involved in reducing nuclear stockpiles and achieving disarmament is reduced. None of that went anywhere.