British Prime Minister Theresa May in London on Wednesday. (Andy Rain/EPA-EFE/Shutterstock)

George J. Mitchell, a former U.S. Senate majority leader (D-Maine), served as independent chairman of the Northern Ireland peace talks.

Twenty years ago, the governments of the United Kingdom and Ireland and eight Northern Ireland political parties entered into the Good Friday Agreement. I was serving as independent chairman for the talks, and when I announced the agreement, I described it as historic, which it was. But I also said that, by itself, the agreement did not guarantee peace, stability or reconciliation. It made them possible.

But achieving and sustaining those lofty goals would require of future leaders the vision and courage demonstrated by the leaders of Northern Ireland in 1998. They continue to confront challenges, the most recent, and most serious, being the decision of the United Kingdom to leave the European Union.

In the long and complex negotiations between Britain and the E.U. over the terms of the separation, a central issue has been the border between Northern Ireland, which is within the United Kingdom, and the Republic of Ireland. For decades, as the bitter sectarian conflict known as the Troubles raged in Northern Ireland, the border was increasingly fortified to limit crossings. When I arrived in Northern Ireland in 1995, it was common to meet people on both sides who lived close to the border but had never crossed it. There was minimal commerce and maximum hostility. In that atmosphere, stereotyping and demonization of “the other” was widespread.

One of the great achievements of the Good Friday Agreement was the opening of the border. For 20 years, as peace has prevailed, cross-border commerce in goods, services and people has risen dramatically, to the benefit of both sides. So widely recognized are the benefits of a “soft” border that a year ago, as they began their final negotiations, the E.U. and the U.K. publicly pledged that, whatever the outcome of those negotiations, they would not include the return of a “hard” border. But, as the Brexit process hurtles to an uncertain conclusion, keeping that promise is proving difficult.

It is a promise that must be kept. A hard border would be in no one’s interest. It would stifle the commerce that has so recently and beneficially flourished, and it could increase the possibility of a resumption of violence. That is impossible to predict with any certainty, but even the possibility should be sufficient to prevent a return to a hard border.

While the difficulties are many, those engaged in the negotiations on behalf of the U.K. and the E.U. should extend themselves to reach a compromise that permits some continued economic and political relations between them. The Bank of England reported that a hard Brexit — a break with the E.U. without a new negotiated agreement in place — would have a severe adverse effect on the British economy. And the U.S. Federal Reserve warned of the risk of an escalation of trade tension and geopolitical uncertainty that could trigger a sharp decline in asset prices.

That is a risk on both sides of the negotiating table. The hostile reaction of other E.U. member states to the United Kingdom’s insistence on separation is understandable. But they must set aside their hurt feelings and balance their concerns over others leaving. They must concentrate on their long-term self-interest. The E.U. will be significantly diminished by the departure of the United Kingdom, which has the second-largest economy of the E.U. member states and one of the largest military forces, and it is an international power of long standing. The E.U. has special, unique relationships with Norway, Switzerland and Canada. It is difficult to understand why it can’t have one with the United Kingdom.

The United States has a large stake in the outcome, too. A weakened and divided Europe would mean the loss of valuable democratic allies in dealings with large hostile powers and with the massive social, economic and migration upheavals that are occurring and will continue in Africa and Asia. The cooperative efforts by the United States and its allies in Europe and Asia, begun under American leadership in the aftermath of World War II, have been beneficial to all who participated in them.

The United States’ ties with Europe pre-date the establishment of our country. We gained our independence from England by revolution, but we retained England’s language, the spirit of its laws and many of its customs. Although our early relations were hostile, over time the two countries formed what remains a “special relationship.”

As our nation grew to settle a vast continent, we welcomed millions of immigrants from England, Ireland, Germany, France, Italy, Greece, Poland, Scandinavia and many more places. We share deep bonds with Europe, not just legal relationships.

While we compete in many ways, we should not think of Europeans primarily as adversaries. They are also our partners and our allies. Although they do not always agree with us, or even among themselves, for the most part they admire our country and share our values and interests. It is in everyone’s interest that we do all we can — politically, economically, militarily and otherwise — to help the people of Europe remain democratic, united, free and prosperous.