What we are witnessing in Ukraine is the first in what will be a series of moves made by aggressive, avaricious nation-states in the coming decades, and indeed, could become the new norm for conflict escalation around the world, unless the Obama administration develops some new tools for response in a hurry.
Since the Peace of Westphalia ended the Thirty Years War in 1648, European states bought into the notion of sovereignty, or being free from outside interference in their domestic affairs, or the affairs of their nation of peoples. Thus the nation-state was born. Europeans then carried this notion out into the world with them over subsequent centuries; wars were fought between and among nation-states, not individuals, not groups and not nations. On the basis of this fundamental understanding, conflict happened, treaties were negotiated, goods and services were exchanged, and we eventually saw that the majority of international relations were conducted at the nation-state level.
Throughout the 20th Century, what we saw, and what Putin has neatly circumvented in Ukraine, was the idea that aggression comes from one nation-state to another. This occurred, it has been argued by many, in a largely anarchic society of nation-states who were concerned ultimately with self-interest. As a result, nation-states, like the US, became guarantors of sovereignty for themselves first (and those nation-states who were their allies second) on the basis of military strength. This strength was measured by the ability to deter or defeat another nation-state's military. As a result, nation-states acted in response to other nation-states. But in the 21st Century, when the aggressor isn't a nation-state, the options for military response is effectively muted.
This is Putin's tactically brilliant maneuver. Ukraine's sovereignty is certainly being assailed by members of the Russian nation, but Ukraine is not being directly assaulted by the Russian nation-state. This makes a definitive, protective, military response by those nation-states who would support Ukrainian sovereignty (or the idea of sovereignty in general) impossible, for against whom can they direct their military efforts? Putin's plausible deniability is clearly a sham, but it is thick enough that no nation-state can act militarily against his nation-state, while members of the Russian nation begin to systematically dismantle Ukrainian sovereignty, first in Crimea, and now in the eastern part of Ukraine. Ultimately the Russian nation-state will benefit from the influx of natural gas, warm water ports and other resources and will pay little to no military cost for these gains. Russia may pay an economic cost through sanctions, but Putin is clearly calculating that economic sanctions with either not last long, or will not be all that costly or both.
In the aftermath of 9-11, the US learned about how a nation-state can respond to the super-empowered individual, and it found the experience to be messy, largely unsatisfying, and costly. Now the US (and all nation-states) will have to figure out how to respond to the diffuse aggression of nations of people within foreign states. This isn't a new problem: witness how China has occupied Tibet. The world of nation-states will be challenged to come up with new solutions to this problem in the coming decades, lest they wake up one morning to discover that their sovereignty is merely a thought from the past.