Ukraine Starts US-Russia war

In 2014, Russia took over the Crimean peninsula and gave support to rebels wishing to break off of Ukraine in response to the ousting of the pro-Russia president Viktor Yanukovych. People have some conflicting views on exactly what happened, but the question here is What would have happened if Russia flat out invaded the whole country, no "little green men" or anything like that, triggering a war with the USA and Europe. For the sake of discussion, the war takes place only in Ukraine, no nukes, strategic bombing, or anything like that, but cyber attacks are probable. What other countries will get involved? What will it look like? Will it be officially declared, or remain a "policing action". What will the effects on the world be?
 
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