CNN  — 

Russian President Vladimir Putin on Thursday began a military operation in Ukraine after weeks of warnings by Western powers that such an attack was imminent.

But Russia’s invasion of Ukraine was also precipitated by years of rising tensions. Below are six maps that help illustrate the events that led to this moment.

NATO since the Cold War

Since the end of the Cold War, more countries have been joining the North Atlantic Treaty Organization, or NATO, pushing the bloc eastward.

In 2008, NATO declared its intention to offer membership to Ukraine – a former cornerstone of the Soviet Union – some day in the distant future. But Russia considered that crossing a red line.

This map shows NATO’s expansion eastward in Europe. Many countries in western Europe joined prior to 1991 and the end of the Cold War, while those to the east – including the former Soviet Baltic republics of Estonia, Latvia and Lithuania – joined in the years since.

Putin has indicated he sees NATO’s expansion as an existential threat, and the prospect of Ukraine joining the Western military alliance a “hostile act.” He has emphasized his view that Ukraine is part of Russia, culturally, linguistically and politically, and Russia asked amid recent tensions for guarantees the European and North American defense alliance wouldn’t expand further east.

The United States and NATO have resisted. The alliance’s “open door policy” states that any European country ready and willing to undertake the commitments and obligations of membership is welcome to apply for membership. And since the end of the Cold War, more than a dozen countries from the former Eastern bloc have joined.

Annexing part of Ukraine

In early 2014, mass protests in the Ukrainian capital forced out a Russian-backed president after he refused to sign a historic political and trade agreement with the European Union.

Russia responded by annexing the Ukrainian peninsula of Crimea, highlighted in red along the Black Sea’s northern coast in the map below, and fomenting a separatist rebellion that seized control of part of the Donbas region along Ukraine’s southeastern border with Russia.

Despite a ceasefire agreement in 2015, the two sides have not seen a stable peace, and the front line has barely moved since.

Inflamed tensions

In the eight years since, Moscow has been accused of engaging in hybrid warfare against Ukraine, using cyberattacks, economic pressure and propaganda to whip up discord. Those tactics have escalated in recent months, and in early February the US State Department claimed Putin was preparing a false-flag operation to create “a pretext for an invasion.”

Those efforts have also manifested themselves on the ground, as seen in this map, which illustrates the buildup of Russia’s military presence around Ukraine in the past year. Russia has amassed more than 150,000 troops, as well as equipment and artillery, on the country’s doorstep.

As the situation on Ukraine’s border has intensified, NATO has raised the readiness of its rapid response force, while member countries put troops on standby and deployed battalions, planes and ships to defend member states in the region. The buildup of these resources, seen in this map, includes thousands of US troops deployed in Poland.

Russian invasion

On February 21, Putin said he officially recognized the self-declared Donetsk and Luhansk People’s Republics (DNR and LNR), highlighted in the map below in red, in eastern Ukraine’s Donbas region, ordering the deployment of Russian troops there on the pretext of protecting the local population.

The territory recognized by Putin extended beyond the areas controlled by pro-Russian separatists, raising red flags about Russia’s intended creep into Ukraine.

Hours before dawn on Thursday, February 24, Russia’s assault on Ukraine finally began with a series of missile attacks and the use of long-range artillery. This map shows the locations of reported attacks and explosions as of Thursday night, including in and around major cities such as the Ukrainian capital of Kyiv, Kharkiv in the east and Odessa in the south, among others.

Head here for the latest updates on Russia’s invasion of Ukraine.