The Israeli barrier between the Palestinian refugee camp of Shuafat, right, and the Jewish neighborhood of Pisgat Ze’ev that is considered by many as a Jewish settlement in east Jerusalem in 2014. (Abir Sultan/European Pressphoto Agency)

A half-century ago this week, Israel embarked on what its leaders believed was a preemptive strike in Egypt, beginning a short and intense regional conflict that also swept up Jordan and Syria. Over the course of the Six-Day War, Israel captured and occupied the Sinai Peninsula, West Bank, Gaza Strip and Golan Heights, dramatically changing the territorial facts of the Middle East.

Only, not quite. The pre-1967 borders have been surprisingly resilient to the realities created by the war.

Despite a dubious legal basis and a history as an actual border for fewer than two decades, the pre-1967 border is still the primary reference line in any discussions about a two-state solution between Israel and the Palestinians. After its 1979 peace agreement with Egypt, Israel withdrew to this line, and the disputes in the failed negotiations between Israel and Syria in the 1990s revolved primarily around the exact placement of the pre-1967 line. Implicitly, it was also the base for Israel’s 2005 unilateral withdrawal from the Gaza Strip. International actors as diverse as the European Union, Russia and the Arab League agree on its validity.

Until the current administration, which seems still ambivalent on the matter, this line was also the base for negotiations for both Republican and Democratic U.S. administrations. And for the vast majority of the people around the world who protest the Israeli occupations of Palestinian land, the occupation means anything beyond the 1967 lines. So why this resilience?

How was the 1967 line formed?

Often abbreviated as “the 1967 line,” or referred to as the Green Line, the pre-1967 border is a line of armistice agreed to in 1949 at the end of a war that Israelis call the War of Independence and Arabs term the Nakba, or disaster. These agreements established a de facto line of control at the end of the war that differed significantly from the borders agreed upon by the United Nations’ 1947 partition resolution.

The armistice lines were explicitly meant to serve as temporary lines until permanent lines were determined in peace negotiations, which of course never materialized before the 1967 war. So while the original 1947 U.N. partition resolution has a stronger basis for international law, the temporary armistice line became the recognized international border of Israel with its neighbors.

Validation through recognition, immigration and administration

Why did the majority of the international community — if not neighboring Arab states — recognize the 1967 line? The reason was simple: Regardless of legal argument, states with the military power to take over a territory and the administrative capacity to control it have historically been recognized as legitimate. These were just the rules of the game.

Think, for example, of the United States’ territorial grab in the aftermath of the Mexican American War of 1846 to 1848. Or of the constant territorial changes of European states during the 17th to 19th centuries. Indeed, Israel — through massive Jewish immigration, the establishment of towns and villages throughout the territories of the newly created state and its military control of the new borders — managed to hold the armistice line and win international legitimacy.

What is different about Israel’s 1967 territorial acquisitions?

All of the factors that prompted the international community to recognize supposedly temporary armistice lines remain, but the post-1967 territory gains have been far more contentious. Through its settlement project, Israel is still creating “facts on the ground,” and it possesses an even more powerful military and sophisticated intelligence operation. Historically, these factors should allow it to effectively control the territory it acquired through war and be internationally recognized.

Today, no other state in the region is even remotely capable of threatening to evict Israel from the land it took in the 1967 Six-Day War. So why does this pre-1967 borderline remain relevant? How can we explain its remarkable resilience? And why do a majority of the international community recognize the pre-1967 line, and no other, as Israel’s legitimate border?

The changing rules of the territorial game

In the aftermath of World War II, the international community came to adopt new standards around the use of force to alter boundaries, often referred to as the “territorial integrity norm.” In my research, I focus specifically on formal borders and have adopted the term “border fixity norm” to describe the relatively new global consensus against forcibly challenging borders.

Since the early 1950s, territorial grabs by one existing state against another — as opposed to secession — have been rare. In these few cases, the reaction of the international community varies from a decisive military eviction of the occupier — such as in Iraq 1991 annexation of Kuwait — to biting economic sanctions — such as in Russia’s 2014 takeover of Crimea.

In no case have more than a few members of the United Nations recognized the annexation of significant territory of one member by another. As the White House statement in the aftermath of the Russian invasion of Ukraine stated, “In this century, we are long past the days when the international community will stand quietly by while one country forcibly seizes the territory of another.”

Why international norms matter

International norms are made by states and could be undone by them. But this particular norm is one that is particularly resilient as it is well institutionalized not only in the “West” but also in almost every regional organization. And not only in ink but in practice.

Many Israelis point to this discrepancy between the different treatment of Israel’s occupation and what “everybody else is doing” as proof of the international community’s hypocrisy toward the Jewish state. They are wrong. Conquest and annexation of others’ homeland territory is what everybody else was doing, but not anymore. This is why the pre-1967 line lives on, 50 years after it was seemingly scrapped by advancing Israeli troops.

Boaz Atzili is an associate professor at the School of International Service, American University. He is the author of “Good Fences Bad Neighbors: Border Fixity and International Conflict,” (University of Chicago Press, 2012) and a forthcoming book about Israel’s policy toward neighboring states that host violent nonstate actors.