A rocket launches from a S-400 missile system against a dark blue sky
A Russian S-400 air defense system launches.
  • Ukraine has been able to fly surprisingly far into Russia unimpeded, a military expert said.
  • George Barros told BI that Russia's air defenses have been stretched for multiple reasons.
  • Ukraine has destroyed some Russian systems and is targeting more, straining Russia's resources.

Ukraine has been able to strike unusually deep inside Russia because Russian air defenses have been stretched, a warfare expert told Business Insider.

George Barros, a Russia expert for the US-based Institute for the Study of War, said many of Ukraine's drones were able to penetrate past the point where Russian air defenses should be able to stop them.