The war in Ukraine may be entering a new and critical stage with a fresh focus by Russia’s military, a last stand in a key Ukrainian city and the promise of additional firepower from the US to help Ukraine fight back. New focus. Russia’s military is now focused on the disputed eastern portion of Ukraine that was thought to be its objective before the invasion began. British intelligence suggests Ukrainian forces have repelled numerous attempted advances by Russian forces in the area – but US officials warn the Russians may be preparing for a larger assault across southeast Ukraine. Ukraine’s Alamo. Advances by Russians in the south against besieged defenders in the surrounded city of Mariupol, a key supply hub, are dimming hope for Ukraine’s military there after the commander issued a desperate plea for help. Ukraine’s government says women and children are also seeking refuge at a steel plant that is the military’s redoubt. Russian President Vladimir Putin said Thursday his forces would not storm the plant. More weapons coming. A new injection of weapons is now on its way from the US to help Ukraine adapt to a different stage of the conflict. President Joe Biden said the war is in a critical stage and the US is looking for ways to prop up Ukraine’s military. Another $800 million in assistance. On Thursday, Biden announced another roughly $800 million military assistance package to Ukraine – on top of the $800 million announced last week. “We have to accelerate that assistance package to help prepare Ukraine for Russia’s offensive that’s going to be more limited in terms of geography, but not in terms of brutality,” he said, adding that he would soon be asking Congress for more money to help arm Ukraine. Separately, spare parts have helped Ukraine add about 20 airplanes to its air force, a senior US defense official said Wednesday, although it’s not clear where the parts came from. Grand total: It adds up to about $3.4 billion in US assistance to Ukraine since the invasion began. What’s being sent to help in the East? Artillery and long-range systems could be more helpful in the more open terrain. This is the same contested Donbas region where Russian-backed forces have been fighting with the Ukrainian military for years. What happens with all this firepower? It’s not entirely clear. An interesting CNN report focused on how the US loses track of the weaponry it sends to Ukraine. From CNN’s report: In making the decision to send billions of dollars of weapons and equipment into Ukraine, the Biden administration factored in the risk that some of the shipments may ultimately end up in unexpected places, a defense official said. But right now, the official said, the administration views a failure to adequately arm Ukraine as a greater risk. I asked CNN’s Oren Liebermann, who helped write recent stories about American military aid, what to make of all these additional weapons likely headed from the US to Ukraine. Pay attention to the importance of artillery, he told me. It is expected to be a big part of the next shipment, and it was an important (though far smaller) part of the last $800 million package. Why wasn’t more artillery sent to this point? LIEBERMANN: Because artillery was not the type of weapon needed to defend Kyiv. In the swamps and forests of northern Ukraine, not to mention the Chernobyl exclusion zone, artillery was not a critical part of the fight. With the focus now in Southeast Ukraine, artillery and other long-range weaponry absolutely is critical. Is this new technology or a game-changer in the fight? LIEBERMANN: Let’s be clear – artillery isn’t new tech. Maybe the systems are newer and offer a bit more precision or more firepower, but artillery isn’t some newfangled piece of military hardware that’s never been seen before. Quite the contrary – it’s been an integral part of wars for many, many decades, if not a couple centuries at this point. And yet it’s viewed by the US and its partners/allies as one of the most important pieces of weaponry to get to Ukraine … and fast. Ukraine can burn through artillery ammo quickly, so it’s important to get them a very large supply ASAP. How does the Pentagon view its responsibility for these weapons once they’re in Ukraine? LIEBERMANN: That’s not viewed as critical. It’s DoD’s job to get it to the border, then Ukraine takes it from there to wherever they believe it’s needed. Ukraine has a nearly insatiable appetite right now for more weaponry, and that’s what the US is trying to meet, along with the help of other countries. What do we know about what’s been sent so far? Per CNN’s Ellie Kaufman, a senior US defense official told reporters Wednesday that the first of 40,000 Howitzer rounds, a type of artillery ammunition, have arrived in Europe to be sent to Ukraine. These are a part of last week’s $800 million package. The US is training about 50 Ukrainians in a country outside of Ukraine (it’s not clear which one) on how to use the Howitzer rounds. “This is to train the trainers; it’s a smallish number of Ukrainians, little bit more than 50 – they will get trained on how to use the Howitzers and then they’ll be able to go back into Ukraine and train their colleagues,” the official said. The NATO question looms. Finland and Sweden are both now actively considering joining NATO, so if Putin’s plan was to stop the growth of the alliance, it has officially backfired. Russia has warned such moves could lead to a more aggressive stance with regard to its hypersonic or nuclear weapons. Saber-rattling. An intercontinental ballistic missile test by Russia on Wednesday is a message to countries that try to threaten it, according to Putin. Walkout. Russia was further isolated from the world community after Western finance officials, including the US Treasury secretary, walked out of a closed-door meeting of the Group of 20 industrialized nations in Washington rather than hear a Russian presentation. What is the nuclear threat in Ukraine? CNN’s Barbara Starr and Zachary Cohen report the US military is keeping a constant watch on Russia’s nuclear arsenal. The US has not seen any indication Russia has moved to prepare its nuclear arsenal for use, and Starr and Cohen report that US officials still feel there is only a very remote likelihood Putin would cross that line. But Ukrainian President Volodymyr Zelensky warned last week the world should prepare for the possibility Putin could use nuclear weapons. From Starr and Cohen: Highly classified US military plans continue to be updated about what everyone believes is an almost unthinkable scenario: the use of a nuclear weapon. The US military “has planned all this out,” a senior defense official said. Endnote: Life on the front lines in the Russian army. CNN’s Phil Black went to an abandoned Russian military camp outside Kyiv. When the soldiers failed to take Ukraine’s capital, they were left to live in primitively dug fox holes. Black talks to civilians who were captured and tortured by the Russians as well as a priest who was asked to bury people they killed. See his video report here. This story has been updated with additional developments Thursday.