Depends upon detection mechanism and reaction time of system in question. Not wise to station ballistic missiles near border and they take time to reach a target.
S-400 systems will certainly upset strategic balance in the region.
This arms-race will get out of hand, if Pakistani economic woes are not fixed.
That is not the fault of S-400 system.
US has technological superiority over every country in reality. Russia and China are near-peer adversaries due to their capacity to strike at US mainland but not in conventional terms.
Tomahawk cruise missiles are very capable at present. Read about Block IV variant.
And it is quiet possible that US spoofed Russian defenses in the process.
My friend,
Perhaps you are not paying attention and unaware of the concept of "midcourse discrimination." Watch this video to understand the entire kill chain process:
The network is incredibly vast, sophisticated and powerful - unparalleled in capabilities.
GMD EKV [in itself] is extremely fast, maneuverable and equipped with multi-color electro-optical and infrared sensors to discriminate actual warhead from decoys [i.e. threat cloud phenomenon], get a lock on the warhead and collide with it.
The ICBM-class target that was intercepted in a live test in 2017 was actually MiRV'ed and the purpose of the test was to verify execution of "midcourse discrimination" protocols.
Writing is on the wall:
"This is the first full-fledged intercept test of the system since the successful intercept in June 2014 (FTG-06b). Dubbed “FTG-15,” today’s event involved an ICBM-range target launched from the Kwajalein Atoll. Multiple sensor systems, including space-based infrared satellites and likely an Aegis SPY-1 radar, detected the missile and tracked its location. The Sea-based X-band Radar (SBX) also surveilled the missile, providing high resolution imagery which allowed the GMD system to discriminate the missile’s warhead from its accompanying debris.
Using this data, a GBI was launched from Vandenberg AFB. Once in space, the GBI released its Exoatmospheric Kill Vehicle (EKV), which identified the warhead with its onboard electro-optical and infrared sensors, diverted into the warhead’s path, and collided with it."
Accompanying debris - interesting choice of words. These were the decoys (i.e. threat cloud phenomenon). The ICBM-class target was designed to simulate modern-era ICBM capability.
In the video that they released for public consumption, they omitted lot of detail. Watch the aforementioned animation - it show you the complete picture.
44 GMD interceptors active as of 2018 [24/7 basis]. Their numbers will be expanded to 100 in near future.
I am actually surprised that MDA disclosed this much. There was no need.
And how do you know? Through some journalist piece? Journalists receive instructions from their bosses in regards to disclosure of information. Many are not aware of actual facts or keep mum. Spread of disinformation is [deliberate] sometimes, so the public and observers keep guessing.
So where was the aforementioned ICBM exactly heading that day?
Where-ever it was heading, the GMD network got a lock on it when it took-off (i.e. thermal signature), tracked its movement in real-time with a multi-layered setup of sensors (sea, land and space) and the EKV killed the actual warhead (or two EKVs killed two warheads) by getting a lock on it and maneuvering towards it with incredible speed.
Even if the warhead was maneuverable, it could not fool the EKV and/or outmaneuver it.
Please keep in mind that a maneuverable warhead does not behave like a thug trying to dodge cops in a Ford Mustang by changing lanes at every opportunity [mid-flight]. A warhead's maneuverability is to re-adjust its trajectory to a limited extent so that it can reach its target with better precision.
Do you think a Russian maneuverable warhead will change course and head back to Russia after getting a lock from a GMD EKV?
S-400 is nowhere close to GMD network in tracking and killing targets. It is intended to kill TBM, SRBM and MRBM class targets at most. It might intercept a cruise missile as well but not verified. What it can kill, depends upon its detection mechanism in the end and range.
Solution is to find another method of ensuring strategic balance.
More bad news for missile enthusiasts; MOKV is under development and will be tested in the 2020s.
An MOKV will carry multiple EKVs and will be able to intercept multiple ICBMs accordingly.
ICBMs will be obsolete at some point. Nuclear MAD will come to an end.