The Budapest Memorandum committed the signatories not to themselves use force against Ukraine, but it was not a multi-way defensive alliance with all parties which obligated parties to fight against another party who attacked.
https://en.wikipedia.org/wiki/Budapest_Memorandum
According to the three memoranda,[9] Russia, the U.S., and the U.K. confirmed their recognition of Belarus, Kazakhstan, and Ukraine becoming parties to the Treaty on the Non-Proliferation of Nuclear Weapons and effectively removing all Soviet nuclear weapons from their soil, and that they agreed to the following:
Respect the signatory's independence and sovereignty in the existing borders (in accordance with the principles of the CSCE Final Act).[10]
Refrain from the threat or use of force against the territorial integrity or political independence of the signatories to the memorandum, and undertake that none of their weapons will ever be used against these countries, except in cases of self-defense or otherwise in accordance with the Charter of the United Nations.
Refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine, the Republic of Belarus, and Kazakhstan of the rights inherent in its sovereignty and thus to secure advantages of any kind.
Seek immediate Security Council action to provide assistance to the signatory if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".
Not to use nuclear weapons against any non–nuclear-weapon state party to the Treaty on the Non-Proliferation of Nuclear Weapons, except in the case of an attack on themselves, their territories or dependent territories, their armed forces, or their allies, by such a state in association or alliance with a nuclear weapon state.[5]: 169–171 [11][12]
Consult with one another if questions arise regarding those commitments.[13][14]
France and China were not signatories but apparently had similar agreements, which I have not read.
The UK and the US (and I assume China and France, if their agreements had approximately the same content) have fulfilled the Budapest Memorandum commitments
Russia broke her commitment.
EDIT: Well, okay, I'm not sure what China's position has been on Ukraine at the Security Council, though de facto the issue is kind of academic for the moment. Russia holds a permanent UNSC seat, and thus has veto power on the UNSC, and regardless of what countries do there, if it's on defending Ukraine against Russia, I'd bet that Russia will veto it. In the 1950s, the Soviet Union was boycotting the UN and so wasn't present to veto US initiatives on behalf of South Korea, so the US was able to get through stuff to initiate UN authorization to use force on behalf of South Korea after North Korea invaded. But I think that it's probably safe to say that Russia isn't going to let that happen a second time. Also, that dealt with the use of nuclear weapons by an attacker, and that has not happened (and if you recall from earlier in the war, there was a discussion between the US and Russia on what would happen if Russia used nuclear weapons against Ukraine. I don't believe that the material was ever generally-released, but I did see a Polish official announcing that the response would be "conventional" (i.e. non-nuclear), and some discussion that centered around the US possibly authorizing airstrikes on Russian positions in Ukraine).
https://en.wikipedia.org/wiki/Soviet_boycott_of_the_United_Nations
During the Soviet boycott, the Security Council adopted a resolution which allowed for the deployment of UN troops to the Korean war in defense of South Korea against the attacking communist North Korean forces (Resolution 83)
That being said, the US has taken a position that providing arms to a country in a conflict doesn't violate neutrality obligations (which dates back at least to early WW2, where the US was providing arms to the Allies while simultaneously claiming neutrality). Historically, providing preferential access to arms this had not generally been in line with the obligations of neutrality.
The US has also taken the position that providing intelligence to such a party, as it is with Ukraine on Russia, doesn't violate neutrality obligations. Going back to WW2 again, this was how some of the first shooting between Germany and the US started in World War II:
https://en.wikipedia.org/wiki/USS_Greer
At 0840 that morning, Greer, carrying mail and passengers to Iceland, "was informed by a British plane of the presence of a submerged submarine about 10 miles [(16 km)] directly ahead. … Acting on the information from the British plane the Greer proceeded to search for the submarine and at 0920 she located the submarine directly ahead by her underwater sound equipment. The Greer proceeded then to trail the submarine and broadcast the submarine's position. This action, taken by the Greer, was in accordance with her orders, that is, to give out information but not to attack." The British plane continued in the vicinity of the submarine until 1032, but prior to her departure the plane dropped four depth charges in the vicinity of the submarine. The Greer maintained [its] contact until about 1248. During this period (three hours 28 minutes), the Greer manoeuvred so as to keep the submarine ahead. At 1240 the submarine changed course and closed the Greer. At 1245 an impulse bubble (indicating the discharge of a torpedo by the submarine) was sighted close aboard the Greer. At 1249 a torpedo track was sighted crossing the wake of the ship from starboard to port, distant about 100 yards [(100 m)] astern. At this time the Greer lost sound contact with the submarine. At 1300 the Greer started searching for the submarine and at 1512 … the Greer made underwater contact with a submarine. The Greer attacked immediately with depth charges.[6]
That is, the US position was that it could provide arms to the UK and could tell the British where German U-boats were without violating neutrality obligations, as long as it wasn't actually fighting Germany (with the Greer firing back on a self-defense justification after having a torpedo fired at it). Germany wasn't that enthusiastic about that interpretation at the time.
Under the UN Charter, countries are not supposed to engage in war unless (a) they're defending themselves, (b) they're defending a country with which they have a collective security agreement (a military alliance, think NATO or something like that), or (c) the UNSC has given authorization. That being said, there has been somewhat creative interpretation of (c), as with the US arguing that U.N. Resolution 1441 qualified and constituted such an authorization to intervene in Iraq in 2003, which is certainly not a universally-accepted take.