Its possible that three years of fighting between Russia and Ukraine could end at least temporarily in a matter of weeks. Its more likely it could continue for months or even years. US and allied intelligence agencies have concluded Russia is probably not serious about making peace, which means the war would continue. If that happens, who will supply Ukraine with weapons? Possibly the United States. But after the events of the past few weeks, Ukraine and its European allies certainly cant assume American aid will continue.Inside this story The US announced a pause in military aid for Ukraine earlier this week a few days after a disastrous Oval Office meeting between Presidents Donald Trump and Volodymyr Zelenskyy, then suspended intelligence sharing with Ukraine as well. Its still possible the rift might be repaired. During his address to Congress this week, Trump said he had received a letter from Zelenskyy in which the Ukrainian leader said he was ready for peace. Ukraine has also expressed a willingness to sign a deal that imploded last week, which would allow the US to profit from some of the countrys natural resources, though there are also reports that the US may try to link a deal to Ukraine agreeing to a quick ceasefire.In an interview with Fox News on Wednesday, during which he confirmed the pause in intelligence-sharing, CIA Director John Ratcliffe suggested the aid pause would be temporary, and was merely a means to push Ukraine to the negotiating table. In what seems to be a shift in tone at least, Trump threatened new sanctions and tariffs against Russia on Friday unless Moscow agrees to a ceasefire. (The US already has wide-ranging sanctions in place against Russia, and there isnt much trade between the two countries. Its not quite clear what new measures Trump has in mind.) Meanwhile, French President Emmanuel Macron has been developing a plan for a one-month ceasefire to be applied in the air, at sea, and on energy infrastructure, but not along the front lines, in order to test Russias seriousness about agreeing to a more comprehensive peace deal. In a primetime address on Wednesday, Macron told the French public, I want to believe that the US will stand by our side, but we have to be ready for that not to be the case. On Thursday, European leaders convened for a summit in which they discussed plans to increase their own defense spending by as much as $800 billion, including a plan to provide as much as $150 billion in loans to allow countries to purchase crucial systems like air defense and drones. The countries also pledged to provide regular and predictable financial support and increase the amount of military support currently being provided. But if a peace deal doesnt come, will that be enough?Can Europe step in?On paper, the challenge of keeping Ukraine in the fight without US support looks difficult but not impossible. Its true that the US has provided more military aid to Ukraine than any other country, though far less than the figures Trump keeps repeating. Only about 20 percent of the military hardware in Ukraine comes from the United States, with 55 percent produced by Ukraine itself and 25 percent coming from Europe, according to estimates from the Royal United Services Institute (RUSI), a British defense think tank. However, this is more than just a numbers game. The aid provided by the United States includes some systems with no European equivalent. The main example, several experts suggested, is the Patriot missiles used by Ukraine for air defense. The Ukrainians have a variety of systems they use to intercept the missiles, drones, and rockets regularly fired at their cities and critical infrastructure so many, and from so many different sources, that the air defense system as a whole has been nicknamed the petting zoo.But the Ukrainians consider the Patriot, with its advanced radar and long-range, vital for shooting down the largest and most advanced Russian missiles, and no other country makes an equivalent system. This could leave Ukraines cities and energy infrastructure increasingly vulnerable to attack. During a press conference last month, Zelenskyy lamented that Ukraine is already running low. At 3, 4, and 5 am, the commander calls me and says: We are near this city, and we have no missiles for the Patriot systems weve exhausted them There are eight [Russian] missiles incoming, but we have nothing left to intercept them.The ongoing need for missile defense was only further highlighted by a massive barrage of missiles and drones targeting Ukraines critical infrastructure on Thursday night. Oleksandra Ustinova, a Ukrainian member of parliament who chairs the committee overseeing weapons deliveries, pointed out that in contrast to most recent Russian strikes, which used drones, the recent barrages have increasingly involved cruise missiles, which are in more limited supply. I think theyre trying to exhaust our air defense, she said. I see the capacity, and trust me, its not a lot.The US decision to suspend intelligence sharing with Ukraine could also hamper the countrys ability to intercept attacks like these as well as launch long-range strikes of its own into Russia. A lot of your weapons are dependent on the intelligence that is coming from the US, said Ustinova, pointing to the F-16 fighter jets that have been delivered to the Ukrainians, but rely on US-provided radar and targeting data. Other weapons systems, even some of those produced in Europe, rely on US-provided parts, the delivery of which has now been frozen. How fast can help move?Ukraine had actually been receiving more aid than normal for the first few weeks of Trumps presidency, largely because the Biden administration rushed aid that had already been allocated by Congress out the door between the election and Inauguration Day. Experts believe there is likely enough to last until this summer, when the situation will start to become more strained.Some of the aid comes in the form of funding, loans, or grants that Kyiv can use to purchase weapons from US defense contractors. This type of aid seems like the most likely to be restarted, given that it involves contracts already signed with American companies, but for now theres uncertainty in Kyiv about whether these contracts will be honored. In theory, Europe could make up for this funding, especially if EU countries agree to a plan currently being discussed to seize frozen Russian assets in Western banks and transfer them to Ukraine. But theres also uncertainty about whether the administration will allow US defense firms to send weapons to Ukraine, even if they are purchased by other countries. The Europeans can all buy it, but if you decide not to sell it, were done, said Ustinova. With the exception of Patriot missiles, European firms make equivalent products to many of the systems Ukraine has been buying from the US. But the problem is time. Ukraine is only now receiving some of the weapons it ordered in the early months following the Russian invasion. Any country that puts in an order today, you know that they wont get the first one for at least two years, said Mark Cancian, a retired Marine Corps colonel and expert on defense logistics at the Center for Strategic and International Studies. It might be another year after that for the last one to arrive, and that might be even a touch optimistic.The US also provided weapons to Ukraine using whats called drawdown, essentially sending weapons directly from the US militarys own stocks and then allocating money for the Pentagon to purchase replacements. The Trump administration has about $4 billion in funding for drawdown left, but seems unlikely to use it. This is an area where Europe will have a hard time making up for the US. After years of post-war declines, European countries simply dont have the military stockpiles that would allow them to send significant amounts of material to Ukraine. Countries generally dont publicize the extent of their military stockpiles, but, the general understanding is that European countries do not hold nearly enough stocks, said Nick Reynolds, research fellow for land warfare at RUSI, the British think tank. The CEO of Rheinmetall, Germanys largest defense contractor, put it bluntly in a recent interview with the Financial Times: The Europeans and the Ukrainians have nothing in their depots.European countries have dramatically increased defense spending in recent years. Twenty-three of NATOs 32 members now meet the alliances 2 percent of GDP target, up from just three (including the United States) a decade ago. But in the last three years, much of the new equipment and weaponry has already been going to Ukraine. Despite the German government declaring a zeitenwende, or turning point, in its attitude toward national defense after the Ukraine war broke out, the country now has fewer battle-ready brigades than it did when the war broke out, because so much of its equipment has been sent to Ukraine.The Europeans have indeed made progress on munitions, said Katherine Dahlstrand, a fellow at the Center for Strategic and Budgetary Assessments. The question will how quickly they can actually deliver on them, and how quickly Ukraine might need them.The problem is particularly acute when it comes to artillery ammunition. For all the attention given to high-tech systems, from AI-enabled drones to fighter jets over the course of this war, the humble 155-millimeter artillery shell has been arguably the key military system throughout this conflict. Simply put, Ukraine and Russia have been firing them at a rate not seen in any war in decades. Bidens National Security Adviser Jake Sullivan once referred to the challenge of supplying Ukraine with these shells as the wars central math problem. For Ukraine, theres a sad irony to the fact that the US has finally ramped up to the once far-fetched seeming goal of producing 100,000 of these shells per month from fewer than 14,000 before the war began just in time for the US to cut off aid. How much is Europe willing to give?The issues of European countries building up military capabilities for their own defense, and providing for Ukraines defense, are separate issues, even though theyre often discussed together. And in some cases, the two goals may be in conflict. Every rocket launcher, air defense system, or artillery round kept in a warehouse or on a base in Europe is one that could be used on the battlefield in Ukraine. And as the Europeans build up their own capabilities a process likely to accelerate as Trump casts further doubt on US commitment to the NATO alliance these countries may at times even be competing with Ukraine for defense contracts. Denmark made headlines last year when its prime minister announced the country was sending its entire stock of artillery ammunition to Ukraine. Other countries, particularly those closer to the Russian border themselves or those with significant overseas troop deployments, might be reluctant to do something like that. In the long term, there may be enough money and enough firepower to go around. But in the short term, in providing for Ukraines defense, European countries may have to decide what level of risk theyre willing to take when it comes to their own. The increasing importance of drones on the battlefield might seem like some good news for Ukraine: unlike artillery systems, Ukraine produces many of its drones domestically. But, says Reynolds, theres simply no way to innovate away the need for old-fashioned artillery. Artillery has the advantage over drones of being able to put down a very high volume of destructive fire across a large area quite quickly, Reynolds said. Ukraine learned this vividly in early 2024, when due to a long delay in Congress approving a new aid package, the Ukrainians were forced to conserve artillery ammunition, at one point giving the Russians as much as a 10-1 advantage in artillery fire. Due to a lack of ammunition, we lost [the city of] Avdiivka and a number of small settlements, and also suffered significant human losses All this could have been avoided if help had been provided on time, Yehor Cherniev, a Ukrainian member of parliament, told Vox last April. Its still possible that weapons shipments and sales could resume, perhaps in tandem with Ukraine and Russia sitting down to ceasefire talks after highly anticipated US-Ukraine talks in Saudi Arabia this week.But even if that happens, the close partnership between the US and Ukraine has probably been irrevocably damaged, even after Trump leaves office. I think this trust will take years to rebuild, Ustinova, just returned from a trip to visit troops on the front lines, told Vox. Now we understand that in one day, you can just turn your back.See More: