The Withdrawal Agreement that the Government agreed with the EU and was voted through Parliament in early 2020 had the clause relating to the Northern Ireland protocol. Basically this put the Irish border for goods in the Irish Sea. This was crucial to the EU because their fundamental biggest concern is protecting the integrity of the Single Market. This was necessary so that goods could not sneak into the EU (ROI) via the UK using it's internal Single Market. The UK decided not to remain in the Single Market and indeed has said that it wants to be able to diverge from EU consumer standards (eg chlorinated chicken), taxation, state aid etc. This allows for the potential undercutting of EU businesses, thus distorting fair competition, via subsidies and cheaper, less safe or ethically agreeable, goods. Potentially, without checks somewhere, England could be used as a big staging post, depot, assembly point for the rest of the world to get a back door into the EU, as well as British companies, because they could arrive in England, cross the Irish Sea into Northern Ireland and then, unchecked, across into the Republic and then into the rest of the EU. It potentially skirts tariff's as well.
So checks need to be made somewhere. Because of the Belfast Agreement there cannot be border infrastructure between the two parts of Ireland. There is no way of having a smart, frictionless border, though for a long time the government/brexiters denied this reality, so Boris agreed to something Theresa May couldn't/wouldn't, betray the DUP who were no longer needed, and sign up to having a customs and sanitary/phytosanitary inspection border in the Irish Sea (or rather the UK ports shipping between GB and NI). This was hailed as a great deal and the Tory controlled Parliament all voted for it.
It is important to remember also that this bill was introduced before the election, but Parliament was at a stalemate, so the opposition would only agree to an election if No Deal (meaning WA not future Trade Deal) was taken off the table. This meant that at least we had a transition period until the end of December 2020, rather than a cliff edge abruptly on the 31st October 2019.
Now the people who negotiated the legally binding Withdrawal Agreement and the Political Declaration (not legally binding but supposed to be a good faith genuine statement of intent for a basis of a future trading relationship), hailed it as a great deal and a fantastic success for Boris and his negotiating skill, then voted in approval of it, are saying that actually it's a terrible deal and they are now going to pass a domestic law which says they can ignore International Law and do what they want. Furthermore, this was actually a manifesto pledge on which they were elected which they are not just failing to fulfill but doing the very opposite.
They know this and have brazenly admitted it in the HOC.
So, they are quite prepared to say anything, promise anything, to get people to vote for them, then they will deliver, not what was promised, but whatever they actually really want and they are prepared to break the law to achieve their aims. They did this in 2016, it worked, so they are doing it again in 2020.
The Americans are upset because they helped broker the GFA and peace in NI. Crucial to it was the Single Market membership of ROI and UK because it meant that if there were no more border barriers for security checks there were no barriers for people or goods or vehicles either. It allowed Unionists to say they were still in the UK, but in practical terms they were one country. The Irish lobby in the USA is massive. We are in the wrong, breaking our word and the law, there is no way we will have the support of US Senators and Congressmen.
The UK reneging on the Withdrawal Agreement will mean that the EU will have no choice, under the legally binding MFN WTO rules, under it's own legally binding constitution and under numerous other international trade deals it has with other countries that are conditional on the Single Market integrity also, they will have to have border controls and checks at the NI border.
I notice the AG's legal advice is being absolutely slated by the legal profession. It also cites the Gina Miller ruling which has a lot of brexiters responding with glee, but this is a complete misrepresentation or misunderstanding of that ruling.
To keep it simple, Britain is a signatory to the Vienna Convention. It states, quite unequivocally