It would be the largest Russian invasion since World War II, according to President Joe Biden.
If tens of thousands of Russian troops were to launch an incursion into Ukrainian territory, it would change the world, said Biden.
Biden's remarks reflect a growing consensus among experts that any conflict in Ukraine is likely to have far-reaching effects in Europe and beyond.
This is very different from the annexation of the peninsula by the Russians, which was mostly contained within itself, with fighting lasting a little over a month.
Biden made the remarks during a surprise visit to a gift store near the Capitol that sells handmade crafts. The stop was part of a larger effort to show a rise in the number of small businesses.
Russian President Vladimir Putin has been building up Russia's military presence on the Ukrainian border. Putin believes that Russia is protecting itself from NATO. He wants to know if Ukraine will not be admitted as a member.
With more than 100,000 Russian troops and weaponry in place, analysts and policymakers say it is unlikely that even the diplomatic talks between Moscow and NATO can stop Putin from breaching Ukraine's territory.
Biden has promised not to send troops into the country. He directed the Pentagon to prepare 8,500 U.S. soldiers for possible deployment to NATO allied countries in Eastern Europe.
U.S. President Joe Biden talks with shop owner Viboonrattana Honey as he pays a visit to a small store called Honey Made that sells handmade clothing and gifts on Capitol Hill in Washington, U.S., January 25, 2022.Biden was asked if the odds of a Russian invasion had increased or decreased recently.
Biden said it was like reading tea leaves.
Putin has a long record of doublespeak and bluffing. Russia's economy is dependent on energy exports to Europe, and they would likely take a hit from economic sanctions NATO has pledged to impose in response to any attack on Ukraine.
There will be enormous consequences if Putin were to go in and invade the entire country.
As he prepared to buy a sweatshirt, necklace and mug with the face of Vice President Harris on it, he said that it was for Russia, not only in terms of economic consequences and political consequences but enormous consequences worldwide.
NATO's promised economic sanctions are still being negotiated, but they are believed to be unprecedented in their scale and scope, targeting Russia's elites and its defense and industrial sectors.
Biden said he was willing to sanction Putin personally.
It is not clear how much that would accomplish. Putin claims his only source of income is his $140,000 salary, and that he has an apartment and a few vehicles.
Experts believe that Putin is worth billions of dollars and that he uses the global shadow banking system to hide his money.
The West has been watching as Putin has built up his forces and equipment at the Russia-Ukraine border.
Russian officials have repeatedly called on the U.S. to prevent an eastward expansion of NATO.
Russia wants the U.S. to not establish military bases in the territories of former Soviet states that are not already members of NATO.
NATO's article 5 clause states that an attack on one member country is considered an attack on all of them.
All NATO members in the region should be reassured that the alliance is ready to defend them, said Biden on Tuesday.
Biden said there was no reason for anyone in NATO to worry about whether or not NATO would come to their defense.
The president said that the question of what US and NATO troops do next will depend on what Putin does.
Biden stopped for ice cream on his way back to the White House.
The story was contributed to by CNBC'sAmanda Macias.