"Ensure NATO can realiably deter or defeat Russian aggression"
-- Means that making sure NATO (Europe + US alliance) can stop or win against Russia if Russia attacks.
"Even if deterrence fails"
-- Even if Russia is not scared off and actually attacks.
"and the United States is already engaged in a primary conflict in another region"
-- Even if the US military is busy fighting a main war somewhre else. EX in the indo-pacific (against china over taiwan, etc.)
"or must withhold forces to deter a primary conflict in another region"
-- Or even if the US is keeping its tropps and weapons at home to prepare for that other big war, not sending them to Europe.
It just means NATO should be ready to deter or fight Russia even if the US can't send troops because its fighting or preparing for a bigger war somewhere else. But it does not mean stopping NATO to fight against Russia.
