Washington (CNN) — Russia’s unprovoked attack on Ukraine has drawn universal condemnation from Western powers.

Sanctions against Russia and aid to Ukraine came from various directions. But putting troops on the ground in Ukraine, which is not a member of NATO, is a line the United States and other Western allies are unwilling to cross.

US Ambassador to the United Nations Linda Thomas-Greenfield told CNN’s Dana Bash on “State of the Union” on Sunday that the Biden administration had “made clear” that the US would not “lean”.

“We will not harm US troops,” he said.

But what other factors are preventing US troops from entering Ukraine? Here’s what you should know:

Why doesn’t the United States send troops to Ukraine?

Although the US has condemned Russia’s actions at every turn, President Joe Biden has gone to great lengths to make it clear that US troops will not enter Ukraine and confront Russia head-on.

Why is that? As Biden told NBC News earlier this month: “It will be a world war when America and Russia start shooting at each other.” In other words, the entry of the United States into a conflict has the potential to trigger a global war.

Retired Lieutenant General Mark Hertling, CNN’s national security and military analyst, told What Matters on Sunday: “The key to diplomacy is limiting the potential for war. While Russia’s current war of illegal invasion of Ukraine is tragic, chaotic and devastating, it remains a regional conflict. .

“If NATO or the US send troops to Ukraine to help them fight Russia, the dynamics will turn into a multinational conflict with potential global implications because of the status of US and Russian nuclear power. That’s why the US and NATO — and other countries around the world — are trying to influence Ukraine’s success and Russia’s defeat by providing another support,” Hertling said.

What about US troops in Europe?

The United States has deployed thousands of troops across Europe, both before and during Russia’s invasion of Ukraine.

CNN’s Barbara Starr reported Sunday that the more than 4,000 US Army troops deployed to Europe in the interim will now have their duties extended, most likely by several weeks, as part of US efforts to reassure Eastern European allies during the war. current crisis.

But the troops were not there to fight the Russians.

US troops “are not and will not be involved in a conflict with Russia in Ukraine,” Biden said from the White House on Thursday.

Instead, US troops are tasked with defending “our NATO allies and reassuring those allies in the east. As I’ve made clear, the United States will defend every inch of NATO territory with the full force of American strength,” Biden added.

Is there a scenario where the United States goes head-to-head with Russia?

Ukraine is bordered by NATO member countries such as Poland, Slovakia, Hungary and Romania. If Russia threatens any of these countries, the United States, along with France, Germany, Britain and the other 30 members of the NATO alliance, will be bound by Article 5 of the North Atlantic Treaty to respond.

Article 5 guarantees that the resources of the entire alliance can be used to protect any member state. The first and only time it was called upon was after the September 11, 2001 attacks on the US; as a result, NATO allies joined the invasion of Afghanistan.

Will US troops help create a no-fly zone in Ukraine?

The United States will not put American pilots in the air to create a no-fly zone in Ukraine, Thomas-Greenfield said Sunday.

The Biden administration’s stance on keeping US troops away from Ukraine means “nor will we have US troops in the air, but we will work with Ukraine to give them the capability to defend themselves,” he said.

While some Ukrainian officials have called on NATO countries to “close the skies” over Ukraine, establishing a no-fly zone would put the United States in direct contact with the Russian military, something the White House has made clear it is not interested in doing.

How else can the US help Ukraine?

US Secretary of State Antony Blinken said on Saturday he had authorized $350 million in new US military aid to Ukraine.

“Today, as Ukraine bravely and proudly resists Russia’s brutal and unprovoked attack, I have authorized, in accordance with the delegation of the President, the unprecedented withdrawal of the third president of up to $350 million for Ukraine’s immediate defense support,” the top diplomat said. US said in a statement.

The previous drawdowns were $60 million and $250 million, bringing the total over the past year to more than $1 billion, according to an administration official.

In addition, Blinken announced Sunday that the United States will send nearly $54 million in humanitarian aid to Ukraine to help those affected by the Russian invasion.

How does the US punish Russia?

In short, sanctions.

The United States and Western countries have imposed several rounds of sanctions against Russia, targeting its banking, aerospace and technology sectors. These sanctions impose penalties across a wide range of sectors, including:

– Asset freeze for the biggest banks
– Restrictions on debt and equity in important mining, transportation and logistics companies
– Large-scale efforts to close access to important technologies for Russia’s main military and industrial sectors

On Friday, the United States, along with the European Union, Britain and Canada, announced that they would impose direct sanctions on Russian President Vladimir Putin and Foreign Minister Sergey Lavrov.

And on Saturday, the US and European Commission, along with France, Germany, Italy, the UK and Canada, announced that they would exclude certain Russian banks from SWIFT, a highly secure network that connects thousands of financial institutions around the world.

“Sanctions, blockades, economic leverage, building alliances against Putin’s actions, and at the same time supplying Ukraine with weapons and other assistance will hopefully prevent escalation and unintended global consequences,” Hertling said.

What is public opinion saying?

Americans are wary of US intervention in the Russo-Ukrainian conflict, according to a poll conducted ahead of the Russian invasion.

In the AP-NORC poll, only 26% of Americans believe that the US should play an important role in the Russia-Ukraine situation. About half, 52%, said they should play a minor role and another 20% said they should not play a role. A third of Democrats (32%) and 22% of Republicans want the US to play a leading role. Independents are more likely to say that the United States should not play a role; 32% feel that way, compared to 22% of Republicans and 14% of Democrats.

What happened next?

Thomas-Greenfield said on Sunday that the United States “takes nothing off the table” when asked about sanctions against Russia’s energy sector, which have so far not happened.

“We improved as Russia improved, so there’s more to come,” he told Bash.

US and European officials have discussed imposing sanctions on Russia’s central bank, an unprecedented move for an economy as large as Russia. Thomas-Greenfield did not give a precise time for that, but said: “It happened very, very quickly.”

The scale of the central bank’s sanctions is still being discussed and could have a bigger impact than officials have sent, CNN’s Phil Mattingly reported.