Russia’s unprovoked assault on Ukraine, now in its fourth day, has faced universal condemnation from Western powers. Sanctions against Russia and aid to Ukraine have come from many directions. But putting troops on the ground in Ukraine, which is not a member of NATO, is a line that the US and other Western allies have not been willing 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 “has made clear” the US will not “put boots on the ground.” “We’re not going to put American troops in danger,” she said. But what other factors are keeping US troops out of Ukraine? Here’s what you need to know: Why won’t the US send troops into Ukraine? Though the US has condemned Russia’s actions at every chance, President Joe Biden has gone to great lengths to make clear that US forces will not enter Ukraine and engage Russia directly. Why’s that? As Biden told NBC News earlier this month, “That’s a world war when Americans and Russia start shooting at one another.” In other words, the US’ entrance into the conflict has the potential to touch off a global war. Retired Lt. Gen. Mark Hertling, a national security and military analyst for CNN, told What Matters on Sunday, “The key to diplomacy is to limit the potential for war. While the current war of Russian illegal invasion into Ukraine is tragic, chaotic and devastating, it is still a regional conflict.” “If NATO or the US sent troops into Ukraine to help them fight the Russians, the dynamic would shift to a multinational conflict with potential global implications due to the nuclear power status of both US and Russia. Because of that, the US and NATO — and other nations around the world — are attempting to influence the success of Ukraine and the defeat of Russia by providing other types of support,” Hertling said. What about the US troops in Europe? The US has deployed thousands of troops throughout Europe, both before and during Russia’s invasion of Ukraine. CNN’s Barbara Starr reported Sunday that more than 4,000 US Army troops who deployed to Europe on a temporary basis will now have their tour of duty extended – most likely for several weeks – as part of the US effort to reassure east European allies during the current crisis. But those troops are not there to fight the Russians. US forces “are not and will not be engaged in a conflict with Russia in Ukraine,” Biden said from the White House Thursday. Rather, US troops are tasked with defending “our NATO allies and reassure those allies in the east. As I made crystal clear, the United States will defend every inch of NATO territory with the full force of American power,” Biden added. Is there any scenario where the US would engage Russia directly? Ukraine borders the NATO member countries of Poland, Slovakia, Hungary and Romania. If Russia threatened one of these countries, the US – along with France, Germany, the UK and the rest of the 30-member NATO alliance – would be required by Article 5 of the North Atlantic Treaty to respond. Article 5 guarantees that the resources of the whole alliance can be used to protect any single member nation. The first and only time it has been invoked was in the aftermath of the September 11, 2001 attacks on the US; as a result, NATO allies joined the invasion of Afghanistan. Will American troops help create a no-fly zone in Ukraine? The United States will not put US pilots in the air to create a no-fly zone in Ukraine, Thomas-Greenfield said Sunday. The Biden administration’s posture of keeping US forces out of Ukraine means “we’re not going to put American troops in the air as well, but we will work with the Ukrainians to give them the ability to defend themselves,” she said. While some Ukrainian officials have called on NATO countries to “close the sky” over Ukraine, establishing a no-fly zone would put the US in direct engagement with the Russian military which the White House has made clear it is not interested in doing. How else is the US helping Ukraine? Secretary of State Antony Blinken said Saturday he has authorized $350 million in new US military assistance to Ukraine. “Today, as Ukraine fights with courage and pride against Russia’s brutal and unprovoked assault, I have authorized, pursuant to a delegation by the President, an unprecedented third Presidential Drawdown of up to $350 million for immediate support to Ukraine’s defense,” the top US diplomat said in a statement. Previous drawdowns have been for $60 million and $250 million, putting the total over the last year at more than a billion dollars, according to an administration official. Additionally, Blinken announced Sunday that the US is sending nearly $54 million in humanitarian aid to Ukraine to assist those affected by Russia’s invasion. How has the US punished Russia? In a word, sanctions. The US and Western countries have imposed several rounds of sanctions on Russia, targeting its banking, aerospace and technology sectors. These sanctions enact penalties across industries, including: On Friday, the US – along with the European Union, United Kingdom and Canada – announced it would impose sanctions directly on Russian President Vladimir Putin and Foreign Minister Sergey Lavrov. And on Saturday, the US and the European Commission, along with France, Germany, Italy, the United Kingdom and Canada, announced they would expel certain Russian banks from SWIFT, the high-security network that connects thousands of financial institutions around the world. “Sanctions, blockades, economic influence, building of alliances against Putin’s actions while simultaneously supplying Ukraine with weapons and other aide will hopefully prevent escalations and unintended worldwide consequences,” Hertling said. Where does public opinion sit? As we wrote about last week, Americans are wary of US intervention in the Russia-Ukraine conflict, according to polls taken in the run-up to Russia’s invasion. What comes next? Thomas-Greenfield said Sunday that the US has “not taken anything off the table” when asked about targeting the Russian energy sector with sanctions, which so far has not happened. “We’re ramping up as the Russians ramp up, so there’s more to come,” she told Bash. US and European officials have discussed targeting the Russian Central Bank with sanctions, a step without precedent for an economy of Russia’s size. Thomas-Greenfield did not give precise timing for that but said, “This is happening very, very quickly.” The scale of the central bank sanctions are still under discussion and could have even more bite than officials have telegraphed, CNN’s Phil Mattingly has reported. This story has been updated with additional information Sunday.