When journalist Adi Ignatius asked Vladimir Putin what his favorite Beatles song was in late 2007, the Russian leader replied, “Yesterday.” For some commentators, this expression of nostalgia is enough to convince them that Putin is the second coming of Stalin and that the Cold War is ending more than two decades of dormancy.

However, the clash between the U.S. and Russia over Ukraine represents not a restart to the Cold War so much as it represents a contest over the terms of its proper conclusion.

When the Soviet Union was on its deathbed, its leader, Gorbachev, met with high-level diplomats in order to negotiate the future of East Germany. In early 1990, Gorbachev had agreed to a German reunification plan that halted NATO expansion east of West Germany.

However, after having second thoughts, the U.S. offered Gorbachev a new plan several months later -- one that would allow for German unification and the gradual expansion of NATO forces into East Germany by 1994. While Gorbachev agreed to this altered plan, subsequent Russian leaders, including Yeltsin and Putin, believe that the eastward expansion of NATO into Eastern Europe violates the spirit of these talks.

The Russians and Americans likewise contest the nature of the 1994 Memorandum of Understanding, which denuclearized Ukraine and also called on the major powers to “respect the independence and sovereignty and the existing borders of Ukraine” and to “refrain from economic coercion designed to...secure advantages of any kind.”

However, Russia's latest actions, including its economic pressures on Ukraine, its annexation of Crimea, and its support for rebels in the eastern Ukraine -- not to mention similar interventions beyond its borders elsewhere -- show that Russia is a restless and expansionist military power.

Putin has pursued an all-out

modernization of Russia's military, which will cost $750 billion by 2020. The military seeks to increase the number of active-duty personnel to 1 million by that year and plans to augment the number of tanks, helicopters, planes, ships, submarines, and satellites by significant margins. Putin also aims to have 70 percent of the military's equipment and weaponry be “next-generation” grade by this 2020 deadline.

But why has Russia been so quick to use its military power, especially in Georgia in 2008 and in Ukraine in 2014?

One explanation lies in protecting Russia's economic interests -- and especially its energy interests. More than half of Russia’s federal budget revenue comes from petrodollars: the country contains one-fifth of the world’s natural gas reserves, and Russia exports as much oil as Saudi Arabia.

Russia knows that it must keep its neighbors dependent on Russian energy in order to sustain its economy and to maintain the political support of its lavishly rich “oligarchs" and other magnates.

First in 2006 and again in 2009, Russia’s energy conglomerate Gazprom cut its energy supply to Ukraine after disputes over payments, and prior to the recent crisis, Russia demanded billions in energy payments just as Ukraine was negotiating its entry into the European Union.

Russia has used this strategy before to deter countries from developing closer ties with Europe.

In 2013, it threatened to cut energy shipments to its impoverished neighbor Moldova while it was planning a free-trade deal: at the time, Russian Deputy Foreign Minister Dmitry Rogozin callously told Moldovans, “We hope you will not freeze.”

Russia has taken other precautionary measures to keep other countries hooked on its oil and gas supply. It has constructed a pipeline that bypasses Ukrainian territory in order to guarantee its unobstructed shipment of energy to Europe, which currently imports one-third of its natural gas from Russia.

Russia has also tried to scare off investors interested in fracking natural gas reserves in Poland and Ukraine by claiming that it can cause "black stuff" to spout from one's faucet.

However, others attribute Russia’s military chauvinism to Putin's need to garner domestic legitimacy in a country that his chief ideological supporter and long-time svengali,

Vladislav Surkov, once dubbed a "managed" democracy -- one that is notoriously lacking in political freedom or pluralism.

In other words, Putin may be stoking ethnic nationalism domestically and engaging in an aggressive, irredentist, pan-Slavic foreign policy abroad in order to mobilize supporters at home.

When Adi Ignatius spoke to Putin in 2007, Putin lamented that the rapid dissolution of the USSR left many Russians stranded.

“Twenty-five million Soviet citizens who were ethnic Russians found themselves beyond the borders of new Russia," he told Ignatius. "Nobody gave a thought to them. Is it not a tragedy?”

Since taking the helm, Putin has backed separatists all across the former Soviet Union -- from the breakaway Transnistria region in Moldova, to the republics of Abkhazia and South Ossetia in Georgia, to the pro-Russian rebels in southern and eastern Ukraine.

Following the hasty, militarized, and possibly fraudulent referendum that led to Russia's annexation of Crimea, commentators noticed an especially marked change in Putin's rhetoric.

In a two-hour address to the Russian parliament in March 2014, rather than referring to "Rossisskii" – which designates citizens of the Russian Federation, regardless of their ethnicity, Putin claimed to speak and act on behalf of "Russkii" -- ethnic, linguistic, or cultural Russians regardless of where they live.

This ethno-nationalist propaganda has seeped deep into the consciousness of Russian citizens.

For instance, 90 percent of the population has access to the immensely popular TV show, Vesti Nedeli, or News of the Week, hosted by Dmitry Kiselev – a populist figure not unlike America's Glenn Beck. Kiselev proudly boasts that Russia can still turn the U.S. "into radioactive dust," declares that the U.S. orchestrated the anti-Yanukovych Maidan protests in Kiev, and believes that the blood and organs of homosexuals are unfit for use in life-saving medical situations.

Kiselev was recently appointed by Putin to head the state-run media operation, Russia Today.

This paranoia has also manifested itself in the proliferation of conspiracy theorizing about the cause of the Malaysian airline flight shot down near the Russian border in Ukraine. According to one theory, the plane is actually the missing MH370 aircraft. Other outlets have surmised that the Ukrainian military attempted to shoot down what it thought was Putin's plane.

In other words, Russian expansionism and military growth is the result of two interrelated motivations. In order to retain his good standing with the oligarchic class -- who run the Russian bureaucracy, economy, and media -- Putin must coerce his neighbors to maintain their economic and energy linkages to Russia.

Likewise, in order to demonstrate his ethno-nationalist bona fides to the Russian masses, he must occasionally use force to come to the rescue of Russians who lie outside the Federation's post-Soviet borders.

Putin advertised this combination of military aggrandizement and nativist chauvinism when he removed former defense minister Anatoly Serdyukov from office in 2013 in part because Serdyukov preferred to import military equipment from countries like France and Israel rather than improve previous Soviet models and award military contracts to domestic suppliers.

Russia's current course by which it uses its petrodollars to upgrade and deploy its military in ways that defend its economic and energy interests -- a vicious or virtuous cycle depending on one's point of view -- is not a sustainable one.

However, with Putin enjoying record approval ratings at home and with Russia's neighbors slowly pivoting to other suppliers to sate their energy needs, it is difficult to foretell when Russian expansionism will begin to see diminishing returns.