The world has always been a dangerous place. Now it is dangerous in a different way, because the world order that we've known since the end of the Cold War has been radically transformed. All of the institutions that preserved peace and promoted global trade will be weaker - NATO, the EU, NAFTA - and US relationships with other countries will change, too.
Unlike most European countries, the police in the US are decentralized; even the FBI has regional offices. Amending the constitution is a long and tedious process.
Putin saw the Ukrainian revolution as a challenge to him personally, and I think that's why he, in fact, over-reacted. I think his occupation of Crimea and then annexation for him was actually a mistake from Russia's point of view. And then his invasion of Eastern Ukraine was also a mistake. He imagined that he would invade Eastern Ukraine and then eventually split the country in half, and he discovered that in fact, Russian-speaking Ukrainians are not Russians, and they didn't support him.
