Tensions have mounted this week between Ukraine and Russia over deadly protests and apparent Russian military aggression in southern Ukraine. Although Putin's only public comments so far were on Friday, when he said Russia will "continue contacts with partners in Kiev," history shows that leaders of Russia and the former Soviet Union have often used military force along its massive borders.

From Czar Nicholas helping the Hungarian emperor quell rebellion in 1849, to Vladimir Putin invading Georgia in 2008, Moscow often signals its intentions ahead of time.

Russia's justification for such interventions has shifted from Soviet times. Now, the Kremlin claims it is protecting ethnic Russians who live beyond its borders.

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"The language today is different than the Soviet era, which was based on communist ideology and class solidarity. Today, the rhetoric is of protection of co-ethnics," said Ariel Cohen, senior fellow at the Heritage Foundation.

But what does all the tough talk mean? Experts say that until Putin, himself, starts making public statements about suppressing Ukraine's new leadership, it's all just posturing.

But there has been significant posturing.

This week, Russia began military exercises with 150,000 troops along the border with Ukraine, sparking fears of a full-scale invasion. Russia has also given safe haven to Ukraine's deposed leader, Viktor Yanukovych, while armed gunmen took over Crimea's parliament building and raised the Russian flag. And on Friday, Ukrainian leaders said Russian naval troops blocked the airport in Sevastapol in Crimea, an autonomous Ukrainian region located on the northern part of a peninsula bordering the Black Sea.

In Kiev, Ukraine's new leaders on Thursday told Russia to back down.

"I am appealing to the military leadership of the Russian Black Sea fleet," said Olexander Turchinov, acting president since the removal of Viktor Yanukovich last week. "Any military movements, the more so if they are with weapons, beyond the boundaries of this territory (the base) will be seen by us as military aggression."

Soviets on the March

In the days of the Soviet Union, communist rhetoric made the case for military actions to stop rebellions in East Germany in 1953, Hungary in 1956 and Czechoslovakia in 1968.

When 1,000 tanks squashed a nationalist uprising in Budapest, Hungary, in 1956, for example, a radio report from Moscow claimed that the Soviet forces "crushed the forces of reactionary conspiracy against the Hungarian people." Right, they were helping the Hungarians.

In August 1968, 200,000 troops entered Czechoslovakia after pro-Soviet politicians submitted a letter to the Soviet press stating that "right-wing" media were "fomenting a wave of nationalism and chauvinism, and are provoking an anti-communist and anti-Soviet psychosis."

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It formally asked the Soviets to "lend support and assistance with all means at your disposal" to save the Czechoslovak Socialist Republic "from the imminent danger of counterrevolution."

In those days, the so-called Brezhnev Doctrine, named for former Soviet premier Leonid Brezhnev, dictated that the Soviet Union would have the right to intervene to protect the status quo within their sphere of influence, according to James F. Collins, former U.S. ambassador to Russia from 1997 to 2001 and a senior associate at the Carnegie Endowment for International Peace.

"There was this premise that you couldn't go backwards from socialism," Collins said. "It played a role always in the way they approached what was considered a threat."

During the height of the Cold War, the former Soviet Union also became involved in conflict against the United States in small, far-off nations like Angola and Cuba.

In Afghanistan, the Soviet Union became embroiled in a local fight for power between a communist leader and a more centrist government. The Soviet military airlifted troops to the mountainous country in 1979, resulting in a quagmire that didn't end until their forces withdrew 10 years later.

Protestors file through Kiev's Independence Square in the wake of recent violent clashes between opposition protestors and law enforcement officers.Vyshinsky Denis/Corbis

"They were faced with either defending the new frontier, or letting it go and seeing what amounted to a setback, if not chaos on their border," Collins told Discovery News.

Modern Interventions

Since the breakup of the Soviet Union in 1991, Russia has intervened twice: once on its own soil in the conflictive province of Chechnya, and, more recently, in the independent nation of Georgia in 2008.

Chechnya was a troublesome province, with hijackings and raids by armed groups that refused to sign a treaty with the government of Boris Yeltsin in 1991. That led to the first Russian intervention in 1992-94.

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"You ended up with Yeltsin turning to the military," Collins said. "That was more a way of enforcing Moscow’s writ in the country that wasn't doing it."

Since the 90s, the conflict in Chechnya has worsened and the region is a breeding ground for terrorist attacks on civilians throughout Russia.

More recently, Russian President Vladimir Putin has used the protection of Russian speakers or ethnic Russians as a rationale for military action, according to Cohen of the Heritage Foundation.

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That's what occurred in Georgia in 2008, when Russia intervened, supposedly to protect Russian enclaves in South Ossetia and Abkhazia. Bad blood between the Russian and Georgian leaders didn't help either.

Despite Moscow's posturing during the current crisis in Ukraine, several analysts warned that it's not a good idea to pay too much attention to public statements by government officials in Moscow.

"Russian rhetoric is often for domestic consumption," said Nadiya Kravets, a researcher at the Davis Center for Russian Studies at Harvard University. "The ruble is falling and the Russian economy is problematic. Having a foreign policy issue that you can use to distract people is useful politically. We need to acknowledge that."