In general, there is no meaningful difference between a “defensive” arms race and an “offensive” one. George W Bush’s exit from the Anti-Ballistic Missile Treaty led directly to nuclear proliferation and acute risk of war. When one side has a more effective missile defense system, the other side gets nervous, making things more dangerous for everyone, not less. Russia probably wouldn’t have hypersonic missiles today if it weren’t for the US advancing its missile defenses.