Russian S-400 defense systems.Russian Defense Ministry / Handout/Anadolu Agency via Getty ImagesEurope is worried that Russia could attack countries beyond Ukraine.At the same time, Trump has suggested the US would be less involved in helping its NATO allies.Without US support, Europe lacks a key capability needed in a conflict with Russia, experts told BI.If President-elect Donald Trump cuts US military cooperation with Europe, its NATO allies there would lose a key capability needed to resist Russian aggression.Europe relies heavily on the US for the use of aircraft and weaponry to target air defense systems, known as suppression of enemy air defenses, or SEAD.If a military can't execute this critical mission, its aircraft are hindered and vulnerable, unable to strike or protect, leaving its ground forces much more exposed and less effective."One of the most crucial things that Europe is lacking is SEAD," Tim Robinson, a military aviation specialist at the UK's Royal Aeronautical Society, told BI.