Why has the US not been more assertive in confronting Russia in recent years?

Over the past few years, Russia seems to have been slipping back towards an authoritarian system of government, with Putin seemingly cementing his control of the country more and more. Along with this, Russia appears to have become increasingly hostile towards the West and emboldened/aggressive internationally. Some examples (not exhaustive) include:

• Annexation of Crimea
• Alleged use of cyber attacks to disrupt Western powers
• Military assertiveness to assist the incumbent regime in Syria
• Military assistance to prop up Maduro in Venezuela

So, why hasn't the US been more assertive in confronting and pushing back against this new period of Russian aggression?

It seems to me that, during the Cold War, the US was much more willing to confront the Soviet Union, which at the time was much more powerful than Russia currently is. It was also a nuclear-armed state, so the argument that the US is afraid of Russia's nukes doesn't seem to explain it.