You don’t have to guess at the text of the Budapest Memorandum. I will link and quote it, it’s pretty short.

Ukraine, the Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and the United States of America,

Welcoming the Accession of Ukraine to the Treaty on the Non-Proliferation of Nuclear Weapons as a non-nuclear-weapon state,

Taking into account the commitment of Ukraine to eliminate all nuclear weapons from its territory within a specified period of time,

Noting the changes in the world-wide security situation, including the end of the Cold War, which have brought about conditions for deep reductions in nuclear forces,

Confirm the following:

The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America reaffirm their commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to respect the independence and sovereignty and the existing borders of Ukraine.

The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine except in self-defense or otherwise in accordance with the Charter of the United Nations.

The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America reaffirm their commitment to Ukraine, in accordance with the Principles of the CSCE Final Act, to refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine of the rights inherent in its sovereignty and thus to secure advantages of any kind.

The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon state party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used.

The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America reaffirm, in the case of Ukraine, their commitment 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.

Ukraine, The Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and The United States of America will consult in the event a situation arises which raises a question concerning these commitments.

This Memorandum will become applicable upon signature.

Signed in four copies having equal validity in the Ukrainian, English, and Russian languages.

Paraphrased more concisely, since Ukraine was giving up the nuclear weapons on its territory, Russia the UK and the USA agreed to respect Ukraine’s existing borders and sovereignty. They’re not allowed to use their weapons against Ukraine except in self-defense nor to use threat or force to damage Ukraine’s territorial integrity, and not to economically coerce Ukraine either. Obviously this part has been broken by Russia.

If Ukraine is attacked anyway then those parties are to seek immediate UNSC action. They did that, drafted a measure to reaffirm Ukraine’s territorial integrity and that the Crimean referendum held no validity, and although 13/15 of the states then on the UNSC voted in favor it didn’t matter because one of the permanent members vetoed it. Guess who.

Additionally no one is supposed to nuke Ukraine unless attacked by Ukraine in association with a state or states that have nuclear weapons. No one has nuked Ukraine.

Lastly, if Ukraine is attacked all powers are supposed to consult with each other about it. Indeed, there has been much consultation over the past more than a decade.

So technically it has been fulfilled (aside from Russia’s flagrant violation of it) since it was a very weak document in the first place. Actually, the US doesn’t technically even consider what little it does say to be binding because it was never sent to be ratified by Congress.

The US and UK did not contract to prevent the invasion of Ukraine or protect it from all comers.

They promised not to invade or to negatively economically coerce.

Russia brokethe Budapest memorandum. The US & UK did not.

Full details in the other reply