The US went to Iraq to PREVENT Saddam from acquiring WMDs.
US is the most influential and powerful state, but it's not an empire, modern post WW2 US is in fact it's on an ideological crusade against imperialism which many countries around the world formerly subjugated by actual imperialist states owe their current independence.
visit to Ukraine as preventive maintenance
Recently Russia has proven it's unable and/or unwilling to:
1. Bring back rebelious ex-client states into the fold - as proven by Ukraine,
2. Protect allies - as proven by Armenia.
Other countries of the former Soviet Union have already noticed. Moldavia is becoming more and more western-oriented. Central Asian republics are turning to China for support and patronage.
Spheres of influence are not given by granted by God - they are won with hard and soft power. It's increasingly obvious Russia lacks both when faced with any other of its geopolitical rivals (USA, Europe, China).