Complex sentence: Relations between Russia and NATO were strained throughout late 2008, due to Georgia's action in South Ossetia and the overwhelming Russian response and have also been strained in May 2009 when NATO expelled two Russia diplomats over accusations of spying.
Split into simpler sentences: Relations between Russia and NATO became strained in summer 2008 due to Georgia's action in South Ossetia and the overwhelming Russian response. Relations were further strained in May 2009 when NATO expelled two Russia diplomats over accusations of spying.