A former Bush administration national security adviser on Thursday raised concern for the Baltic States as Russian forces move into Ukraine.

"They got to be very scared and very concerned. Remember, when the Russians went into Georgia in 2008, we all said if we don’t deny Putin his tactical objectives and make him pay a strategic cost, today it will be Georgia, tomorrow will be Ukraine, and the day after that it will be the Baltic states. So I think if you live in the Baltic states you are very concerned tonight," former national security adviser Stephen Hadley told "America’s Newsroom."

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"Kaliningrad is detached from the rest of Russia by Belarus but also by Poland and Lithuania. My concern would be, for example, if Putin succeeds in what he is doing in Ukraine he would decide it would be great to connect Kaliningrad back up with Russia via Belarus and that means taking territory from Poland and Lithuania. That means a threat to NATO, invocation of Article 5. That means war. This is a very serious moment for Europe and I think they are increasingly seeing it in that way," he added.

Vladimir President Vladimir Putin in Moscow, Russia, Tuesday, Jan. 25, 2022.

Vladimir President Vladimir Putin in Moscow, Russia, Tuesday, Jan. 25, 2022.  (Sputnik, Kremlin Pool Photo via AP)

Russian forces invaded Ukraine on Thursday by land, air and sea in the largest military attack of one state against another on the European continent since the Second World War. 

Ukraine Foreign Minister Dmytro Kuleba said the country severed diplomatic relations with Russia and called "on all our partners to do the same. By this concrete step you will demonstrate that you stand by Ukraine and categorically reject the most blatant act of aggression in Europe since WWII."

The wide-ranging attack on Ukraine on Thursday hit cities and bases with airstrikes or shelling, as civilians piled into trains and cars to flee. Ukraine's government said Russian tanks and troops rolled across the border in a "full-scale war" that could rewrite the geopolitical order. 

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Hadley, who served under President George W. Bush from 2005 to 2009, noted that if Putin succeeds in seizing Ukraine, he will push further to acquire more territory. 

He told hosts Bill Hemmer and Dana Perino that Putin wants to change the regime in Kyiv and install a pro-Russia government. 

Hadley said President Biden will try to "rally the world" to support Ukraine and tens of thousands more U.S. troops may have to deploy to Europe.

Fox News' Danielle Wallace contributed to this report.