Prime Minister Johnson might have promised to get Brexit done, but the drama around the terms of the United Kingdom’s divorce from Europe is far from finished. Neither, despite recent efforts to depose him with a vote of no confidence, is the prime minister.

Now, as the Queen’s Jubilee is firmly in the rearview mirror and as events speed up in both Northern Ireland and Scotland, Mr. Johnson will have to navigate a series of decisions that hold the potential to alter the boundaries of the kingdom the Crown has tasked him with leading, for now.

Both Boris Johnson’s and Brexit’s fates took turns Monday, as Mr. Johnson’s Northern Ireland Protocol Bill passed its second reading in the House of Commons by a 295 to 221 vote, despite fierce opposition from the European Union abroad, the Labor Party at home, and even some Conservative backbenchers. 

The bill allows Mr. Johnson’s government to unilaterally “fix” elements of the Northern Ireland protocol, which governs the economic relationship between Britain, Northern Ireland, Ireland, and the European Union.   

By the protocol’s terms, Northern Ireland remains part of the European Union single market, even as the protocol conjured a trade border in the Irish Sea for goods entering Northern Ireland from elsewhere in the United Kingdom.

The proposed alterations give businesses selling products in Northern Ireland the choice whether they wish to be governed by British or EU rules, implement procedures to fast track the entry of goods to Northern Ireland from Britain, and abolish the European Court of Justice’s jurisdiction over the protocol.  

In labeling the bill “legal and necessary,” the foreign minister, Elizabeth Truss, signaled that Mr. Johnson, her boss, is intent on upending the protocol, an arrangement that was forged in 2020 to address the status of Northern Ireland in Brexit’s wake.

Mr. Johnson has said he hopes the bill could become law by year’s end. The vote fast-tracked the legislation through Parliament, where it is expected to spend mere days under committee consideration.  

The prime minister’s foes are taking aim against the proposal. The foreign affairs minister for Ireland, Simon Coveney, tweeted, “this Bill is no fix.” Mr. Johnson’s predecessor at 10 Downing Street, Theresa May, pulled no punches: “This bill is not legal in international law, it will not achieve its aims, it will diminish this country in the eyes of the world and I cannot support it.”

Most vociferous of all has been the European Union. That body’s Brexit negotiator, Vice President Maroš Šefčovič, has said “the European Union will not renegotiate the Protocol.” Many in Europe view Mr. Johnson’s bill as an overt breach of international law, given that the protocol has the status of an international treaty.

In defending the bill, Mr. Johnson’s government points to its interests in ensuring the “stable social and political conditions in Northern Ireland, the protection of the Good Friday Agreement.” Northern Ireland’s unionist party, which was bested by its rival Sinn Féin in elections last month, has refused to participate in government unless the protocol is revised.  

While 56 percent of the Northern Irish voted against Brexit, most ardent unionists see any European vestige as a betrayal and are pushing the prime minister toward a harder line with the Europeans. Very different is the attitude of the Irish Taoiseach, Michéal Martin, who labeled Mr. Johnson’s plan “dispiriting” and “an assault on an international agreement.”

The drama in Westminster and Belfast is joined by political rumblings from elsewhere in the United Kingdom, as on Tuesday morning Scotland’s first minister, Nicola Sturgeon, announced plans for a second secession referendum, to be held in October 2023. 

Mrs. Sturgeon told legislators in the devolved Scottish parliament that “the issue of independence cannot be suppressed” and that she would never “allow Scottish democracy to be a prisoner of Boris Johnson or any prime minister.” Her remarks were greeted with raucous applause.

As a legal matter, it is unclear if such a vote can be held without the prime minister’s approval, which is unlikely to be forthcoming. The go ahead, which must come from Downing Street, is a provision known as Section 30, which delivers Holyrood the ability to stage a vote.  

Mr. Johnson’s office has said that “now is not the time” for such a vote and that the “focus of the country should be building a stronger economy.” Mrs. Sturgeon has said the question on the ballot will be: “Should Scotland be an independent country?”

The question of whether independence can make its way to the ballot in the absence of Westminster’s approval has been referred to the Supreme Court. Independence faltered during the 2014 vote by a 55 to 45 percent margin. That, of course, was before Brexit.