Russia has placed more than 100,000 troops near Ukraine’s border and ratcheted up war games in the region.

Moscow denies it is planning an assault, but the United States and its NATO allies fear that Russia is planning for war — and are making preparations of their own.

Here are things to know about the international tensions over Ukraine, a crisis reminiscent of the Cold War.

THE US SAYS NO

The U.S. has made no concessions to the main Russian demands over Ukraine and NATO in a long-awaited written response delivered to Russia on Wednesday in Moscow, Secretary of State Antony Blinken said.

Blinken said the U.S. response, delivered to the Russian Foreign Ministry by U.S. Ambassador to Russia John Sullivan, gave up no ground on “core principles” such as NATO’s open-door membership policy and the alliance’s military presence in Eastern Europe.

Blinken said the document made clear that the U.S. is standing by its oft-stated positions. “There is no change, there will be no change,” he said. But, he said the written response to Russia also contains “serious” offers for a diplomatic path to de-escalate soaring tensions over Ukraine by addressing Russian concerns on other matters.

The U.S. response comes as Russia is warning that it would quickly take “retaliatory measures” if the U.S. and its allies reject its security demands and continue their “aggressive” policies over Ukraine.

Russia has demanded guarantees that NATO never admit Ukraine and other ex-Soviet nations as members and that the alliance roll back troop deployments in former Soviet bloc countries, a region Moscow still views as its sphere of interest.