Russia invaded Ukraine by land, air and sea on Thursday. The attack comes after Russian President Vladimir Putin denied for months that he would invade the country consisting of 44 million people, BBC reports

Here are four things you should know about the conflict between Russia and Ukraine.

Putin wants to "demilitarize" Ukraine.

Prior to the attack, Putin announced that he was setting in motion a "special military operation" in Ukraine that included explosions around cities like Kharkiv in eastern Ukraine and the capital, Kyiv, Vox reports.

Putin asserted that he's not trying to occupy the country but instead wants to "demilitarize" Ukraine, The New York Times reports. According to sources, Putin has expressed disappointment in the breakup of the Soviet Union, which included Russia and Ukraine.

"It's worth acknowledging that the demise of the Soviet Union was the greatest geopolitical catastrophe of the century," Putin said in 2005, NBC News reports. "As for the Russian people, it became a genuine tragedy. Tens of millions of our fellow citizens and countrymen found themselves beyond the fringes of Russian territory."

Russia also wants to prevent Ukraine from joining the European Union and NATO.

The Russian president also despises the North Atlantic Treaty Organization (NATO), a military alliance formed in 1949 by the United States, Canada and other Western European nations after World War II.

The countries ultimately joined forces to provide collective national security against the Soviet Union to prevent the USSR from spreading communist ideology. Now, NATO keeps present-day Russia in check by prohibiting it from attacking countries that have joined NATO, according to the Office of the Historian.

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NATO, which now includes Central and Eastern European nations that were once Soviet states, indicated that former Soviet states Ukraine and Georgia could also join. The proposal irked Putin, who declared it a threat to Russian security, The New York Times reports.

Ukraine has been contemplating joining forces with other alliances.

Ukraine's government has been leaning toward establishing closer ties with the European Union and even wrote into the nation's constitution that it will join NATO and the E.U., according to The Washington Post. However, other member nations are wary of accepting Ukraine into the alliance because of the possibility of having to defend the country against Russia and other opponents, according to the Post.

Additionally, in 2014, the U.S. provided military aid to Ukraine after Russia seized Crimea, NPR reports.

Putin is on a mission to reestablish the Soviet Union as a world power and regain its political and economic standing. He intends to gain the economic resources of Ukraine's industrial and agricultural divisions, Fox News reports.

Putin also sees Ukrainians and Russians as "one people," according to AP News, and he indicated that the West manipulated Ukraine to remove its Russian identity, CNN reports. The Russian president's comments have since appeared to ignore Ukraine's sovereignty.

Russia previously placed its troops on Ukraine's border.

Last fall, Russia deployed troops on the border of Ukraine and Belarus, a former Soviet nation supporting Russia, according to U.S. News. Putin said the positions of the soldiers were being used for training exercises. The U.S. now believes that Russia has up to 190,000 in or near Ukraine, The New York Times reports

The United States and its allies saw the move as a potential invasion, but Putin gave a speech to communicate his reasons for his hostility against Ukraine. He said that Ukraine's leaders were oppressing its Russian-speaking citizens and was sowing seeds of discord and hatred against Russia.

Despite Putin's efforts, Sen. Mitt Romney said the United States and other Western nations can not allow Putin to re-create the Soviet Union.

“He's trying to reestablish what he had before. That can't be allowed to happen,” the Utah Republican said while speaking with NBC’s Meet the Press.