(NewsNation Now) —  As Russia builds up firepower near Ukraine, the world is watching for a possible invasion that could have devastating effects on Eastern Europe.

U.S. officials have warned that Russia could invade Ukraine any day now, although the Kremlin has aggressively denied this. To reach this point of crisis, it has taken many years and much hostility between the two countries.

What are the events that led up to the crisis?

Ukraine gained its independence from the Soviet Union in 1991. At the time, it held the largest nuclear arsenal in the world, according to the Arms Control Association.

By 1996, Ukraine had returned all of its nuclear warheads to Russia in exchange for economic aid and security assurance, following the 1994 Budapest Memorandum on Security Assurances.

In November 2013, former Ukrainian President Viktor Yanukovych decided to reject a deal for greater economic integration with the European Union because of pressure from Moscow, per the Global Conflict Tracker, an interactive guide to ongoing conflicts around the world. Protests in Ukraine’s capital city, Kyiv, ensued, with a violent crackdown by state security forces drawing more protestors and heightening the already tense circumstances.

Yanukovych ending up fleeing the country in 2014, a month before Russian troops took control in Ukraine’s Crimean region in March, the Global Conflict Tracker said.

Russia responded to this by formally annexing Ukraine’s Crimean peninsula.

Russian President Vladimir Putin at the time said this was to protect Russian citizens’ rights in the region. At about the same time, open conflict broke out in eastern Ukraine between supporters of Russia and Ukranians loyal to their own government. According to the Council on Foreign Relations, this was the first time since World War II that a European state annexed another state’s territory. More than 14,000 people have died in the conflict that ensued.

Ukraine, the United States and United Kingdom called the annexation a violation of the Budapest Memorandum.

There was an effort to broker a cessation of the violence in February 2015 between France, Germany, Russia and Ukraine, but efforts to reach a diplomatic settlement and resolve the conflict have been unsuccessful, the Global Conflict Tracker said.

What does Putin want in Ukraine?

Putin has devoted his presidency to restoring Moscow’s influence and reasserting Russia as a global power since the breakup of the Soviet Union, an event he has called the “greatest geopolitical catastrophe of the last century.”

Part of doing this could include regaining control over Ukraine.

Putin has said numerous times that the Russians and Ukrainians are “one people,” and has noted that the Crimea was formerly Russian, and was arbitrarily granted to Ukraine by Communist leaders under the Soviet Union.

With the breakup of the Soviet Union back in the ’90s, Russia lost a country it had been linked to since the ninth century. Both countries speak closely related languages, and many Russian citizens still feel a connection with Ukraine. This being the case, Putin has said that Ukraine’s hopes of joining NATO would be a red line for him, as Russian leaders have been fearful of the alliance’s eastward expansion.

Ukraine is not currently a member of NATO but leaders of the western alliance have refused to rule out the idea that it could eventually become a member.

Since the Cold War, NATO has expanded greatly, which Russia sees as a threatening encroachment toward its borders.

In fact, the Russian president once warned William J. Burns, who is now the director of the CIA, that including the Ukraine in NATO would be a “hostile act” toward Russia, the Council on Foreign Relations said. NATO, for its part, has said that it poses no threat to Russia.

“NATO is a defensive alliance, whose purpose is to protect our members,” the alliance said on its website.

The United States and its European allies have rejected Russia’s demands to bar Ukraine from ever joining NATO, as well as several other security demands, such as halting deployments near Russian borders and rolling back forces from Eastern Europe, which has only fueled the fire further.

What’s happening now?

Currently, Russia has amassed more than 150,000 troops east, north and south of Ukraine, and while there have been no indications of a significant withdrawal of those forces, there have been some gestures from Moscow suggesting that Europe could avoid a full-blown war.

Russia said Wednesday that it is returning more troops and weapons to bases, but NATO Secretary-General Jens Stoltenberg said the military organization does not see any sign that Moscow is decreasing its troop levels around Ukraine. In fact, a U.S. official said later on Wednesday Russia actually added 7,000 more troops near Ukraine’s border.

The day before, Biden gave a status update on the ongoing situation, saying “an invasion remains distinctly possible.” During this speech, Biden warned that if Russia invades Ukraine, the U.S. “will rally the world to oppose its aggression.”

The human cost for Ukraine and strategic cost for Russia would be “immense,” should an invasion happen, the president stressed.

Russian statements claiming the withdrawal of troops have lowered the political temperature after weeks of high tensions. But trust remains elusive between the East and West.

Throughout the ever-evolving situation, Ukrainian President Volodymyr Zelenskyy has been trying to project calm, as well as strength. On Wednesday, he declared a “Day of National Unity.”

“We are united by a desire to happily live in peace,” Zelenskyy said in an address to the nation earlier in the day. “We can defend our home only if we stay united.”

A series of meetings are planned in Europe this week, amid the Ukraine crisis: there’s the NATO defense ministers gathering, a summit of EU leaders starting Thursday, and the Munich Security Conference starting Friday.

The Associated Press and Reuters contributed to this report.