Concerns have been raised across Europe about a recent buildup of Russian troops along Ukraine’s border. The fear is that Russian President Vladimir Putin is planning a repeat of 2014, when Russian forces annexed the Crimean peninsula and supported a separatist, pro-Russian insurgency in eastern Ukraine’s Donbas region.

Moscow denies any intention of invading. However, US officials have stated that such a move is possible—as President Biden reiterated on Wednesday—and that more military equipment, including tanks and rocket launchers, has been moved westward toward Ukraine in the last week. Mr. Biden said that any military move would be met in a calibrated way, and suggested that Western nations weren’t moving in step on how to respond to what he described as a “minor incursion,” alarming Ukrainian leaders before the White House began rolling back his remarks.

The US has ruled out sending combat troops to Ukraine, but Mr. Biden has warned Mr. Putin that a military invasion of Ukraine would be met with severe economic consequences.

Last week’s talks to de-escalate the crisis failed to bridge the gap. As Secretary of State Antony Blinken travels to Europe to meet with Ukrainian President Volodymyr Zelensky and Russia’s foreign minister, US officials said Tuesday that Washington is preparing financial sanctions against pro-Russian separatists in eastern Ukraine.

Russia hasn’t revealed much about the size of its troop deployment on Ukraine’s border. US officials, on the other hand, say Mr. Putin is assembling a force of 100,000 troops, allowing him to order an invasion by early 2022. The officials say the Russian military buildup differs from an earlier massing of troops in the spring, citing new intelligence reports that include images from spy satellites. They estimate that once completed, Russia’s deployment in the area will be twice as large. Russia has also begun a rapid mobilization of reservists, as well as the deployment of troops and military assets to ally Belarus, which shares a border with Ukraine.

The US has repeatedly briefed its allies on the possibility of such a move, and Mr. Blinken warned last month of economic sanctions against Russia in such an event. Meanwhile, the White House is reviewing its options for responding to any Russian offensive, which could range from increased military support for Ukraine to additional diplomatic efforts to de-escalate any potential situation.

On January 10, Washington and Moscow began security talks in Geneva. Russian officials concentrated their efforts on persuading the United States to agree that NATO’s eastward expansion should be halted, but they were unsuccessful. A subsequent NATO-Russia meeting was similarly inconclusive, and the US resisted Russia’s attempts to prevent more countries from joining the alliance.

NATO has warned that if Russia launches a new military incursion in Ukraine, its members will face significant costs, while also highlighting the organization’s support for Ukraine, which is not a member of the alliance and thus is not covered by its mutual defense treaty.

When Ukraine was a part of the Soviet Union, it was a valuable resource. Its fertile farmland supplied much of the wheat consumed in the former USSR, and it was a major industrial center. Its vast plains served as a buffer between European powers and Russia’s hinterland. There are also historical, cultural, and linguistic ties that predate the 18th-century rise of the Russian empire. Following the fall of the Iron Curtain, Moscow continued to regard Ukraine as a critical geopolitical space, and it has been watching the growing pro-Western sentiment in the country with concern.

Following the removal of a pro-Russian president from office in Kyiv and other cities, Russia annexed Crimea, home to its Black Sea fleet, and began supporting pro-Russia militias in eastern Ukraine, though it has denied sending troops and equipment to reinforce the rebels.

Mr. Zelensky has stated that Russia’s troop movements are sending a “very dangerous” signal, and that Ukrainian forces are ready to repel any incursion. His foreign minister, Dmytro Kuleba, stated that any attack on Ukraine, minor or major, should be treated equally by the United States and other Western countries, and that Mr. Biden risks underestimating the scope of Russian aggression.