Quite a banquet will take place Thursday evening at London to mark the centenary of the Balfour Declaration. That’s the letter in which Britain announced its support for the establishment in Palestine of what became the state of Israel.

The letter was sent a century ago to a leader of British Jewry, Lord Rothschild, from Britain’s foreign minister, Lord Balfour. The nub: “His Majesty’s Government views with favor the establishment in Palestine of a national home for the Jewish people.”

Its enormous impact was the acknowledgment by the world’s then-mightiest empire of the legitimacy of Jewish claims to Israel. That the claims went back to biblical times was marked in the meeting where Balfour first began to grasp the basis of Zionism.

The meeting, which took place at a hotel in Piccadilly, was with Chaim Weizmann, who would eventually become the first president of Israel. Balfour, then a former prime minister, was running again for Parliament.

Balfour and Weizmann talked about the founder of political Zionism, Theodor Herzl, and the Zionists’ hostility to an offer of a Jewish homeland in Uganda. Weizmann said that if Moses had heard the Zionists considering that offer, he’d have broken the tablets again.

“Mr. Balfour,” Weizmann said, “supposing I were to offer you Paris instead of London, would you take it?”

“But, Dr. Weizmann, we have London,” Balfour replied.

“That is true,” Weizmann responded. “But we had Jerusalem when London was a marsh.”

Balfour leaned back and eyed Weizmann, who was a chemist with dreams of Zion. Then, Weizmann recalled in his memoirs, Balfour asked: “Are there many Jews who think like you?”

“I believe,” Weizmann replied, “I speak the mind of millions.” He meant the millions who for centuries clung to the promise, recorded in Genesis, of God to Abraham and his son Isaac and grandson Jacob.

It would take Balfour nearly a decade to get back into high office. The eruption of a world war brought into focus the geopolitical logic of the Jewish state. Speaking again for the world’s greatest empire, Balfour had a huge impact.

“With one step,” the London Jewish Chronicle wrote, “the Jewish cause has made a great bound forward. It is the perceptible lifting of the cloud of centuries.” It wrote that “the Jew” was to be given a chance: “The day of his exile is to be ended.”

The declaration, Caroline Glick wrote this week in the Jerusalem Post, “didn’t change the way non-Jews felt about the Jews. It empowered the Jews to change their fate. And it gave license to the nations of the world to support them.”

After World War One, the Balfour Declaration became a formal part of Britain’s League of Nations mandate in Palestine. All the major powers grasped what Balfour had set in motion.

Between the two world wars, though, anti-Zionist elements, including Britons in Palestine, helped foment an Arab resistance. Britain itself began to restrict Jewish immigration into the very land that Balfour had envisioned as the Jewish homeland.

It would take a second world war for Israel to be brought into being. In 1947, the United Nations finally resolved — in a vote at Lake Success here in New York — to partition Palestine into two states, including a Jewish one.

Six months later, Israel declared its independence. It has since become a true power, with the 34th highest GDP per capita and a vibrant agricultural and high-tech economy. It has a top-flight military and is bristling with colleges and culture.

Yet even now, Balfour is a crux, a point made this week by journalist Rick Richman. He notes that the peace process is stuck not on the 1967 issue (borders) or a 1948 issue (refugees) but the 1917 issue — the Arab rejection of any Jewish sovereignty in the land of Israel.

No doubt, as the dignitaries gather in London, protesters will be doing their best outside. (Labor Party leader Jeremy Corbyn is boycotting the banquet.) The protests themselves attest to the importance of the Balfour letter.

The Balfour Declaration is, after all, precisely about recognizing the legitimacy that Israel has always inherently had. The Balfour letter merely acknowledged the justice of the Jewish cause. The point is hugely important in the modern revival of the Jewish state, and bears reinforcing at every turn — the Jews’ right to self-determination in their ancient land was there all along. As Lord Balfour came to see.

This column first appeared in the New York Post.