When Bush abandoned the ABM Treaty in order to build the NMD system, the general expectation in the West was that Russia would sooner or later develop countermeasures ensuring that the development of NMD would not negatively affect its own nuclear deterrence capabilities. By the same token, when the Bush administration, under its Nuclear Posture Review of 2002, lowered the nuclear threshold and publicly considered developing tactical low-yield nuclear weapons capable of penetrating deep bunkers, Moscow did not overlook the possibility that such bunker-busters might be used against Russia's command and control, notwithstanding public assurances from Washington that they were aimed at destroying the capabilities of the so-called rogue states.
Russia also made clear its new perspectives regarding the use of nuclear weapons. The 2000 version of its national-security concept lowered the nuclear threshold by stating that Russia no longer envisaged the use of nuclear weapons reserved solely for extreme situations. Instead, nuclear weapons may be used in small-scale wars that do not necessarily threaten Russia's existence. That was clearly aimed at leveling the playing field in view of an unswerving and unambiguous advantage that the US military had demonstrated in conventional warfare in the Gulf War of 1991, and military campaigns in Bosnia and Kosovo.
To underscore the point that Russia viewed the US withdrawal from the ABM Treaty as a potentially serious threat to the deterrence-related credibility of its nuclear weapons, Moscow declared that even if START II (the Strategic Arms Reduction Treaty) were to be ratified by the duma (parliament), the option of multiple-heading intercontinental ballistic missiles would not be removed completely. In other words, Russia would continue to use multiple-warhead intercontinental ballistic missiles if the US deployed an NMD system. However, making significant breakthroughs in ballistic-missile technology that would overwhelm the NMD system to the extent of rendering it strategically meaningless was an option that Moscow badly needed.
Diplomacy is a weird way to do business, but it was invented for a reason. Aw shucks diplomacy, where everyone always believes the US because no-one can doubt the word of the US, has its limitations.
One limitation is that when you develop a defensive weapons system, others tend to develop a new offensive system. The reports of NMD successes have been less than stellar. The technical questions about NMD are fun, as are the technical questions about this new Russian missile. What's not funny at all is a president and an administration so devoid of imagination that they did not see this development coming when they denounced a treaty that was working well in order to pursue a weapons system to end all weapons systems.