Israeli police officers block protestors from reaching the religious group “Women Of The Wall” at Jerusalem’s Western Wall in 2013. | Uriel Sinai/Stringer

He’s trying to keep the ?? in the Middle East.

The US is walking a fine line in one of the most volatile regions on the planet.

On Thursday, aides to President Trump said the Donald had signed an official order keeping the US Embassy in Israel in the city of Tel Aviv. That means Trump broke his promise to move the embassy to Jerusalem.

This whole embassy situation has kicked up a lot of controversy. So what’s all the fuss about?

In short, Jerusalem is ground zero for the Israel-Palestinian conflict, and every viable peace plan has specified that the eastern half of the city would serve as the capital of a future Palestinian state. If the US moves its embassy from Tel Aviv to Jerusalem, it would be affirming that Jerusalem is the capital of Israel.

That would make achieving peace in one of the most complicated places on Earth a lot more complicated.

Jerusalem has always been a sacred, complex city, home to many different peoples and ruled by many different powers. It’s the birthplace of Judaism and Christianity, and is home to some of Islam’s holiest places. In other words, the city is important to about half of humanity.

As a not-so-surprising result, people have long wanted to preserve Jerusalem’s international character and prevent it from being controlled by any one group. When the League of Nations gave Britain a mandate over Palestine in 1922, for example, it decreed that Jerusalem would be a neutral city, controlled by neither Jews nor Arabs. The UN did the same thing in 1947 when it divided Palestine (which had previously been part of the Ottoman Empire) into two nations, one for Jews and one for Arabs.

But the idea of a neutral Jerusalem has been weakened by war and politics ever since. In 1947?—?the same year the UN attempted to partition the country?—?war erupted between Jews and Arabs. The war gave birth to the country of Israel as we know it today, but left Jerusalem divided, with Jordan (a mostly Muslim country) controlling the eastern half and Israel (a mostly Jewish country) controlling the western half.

The balance of power in Jerusalem was upset again in 1967, when another war broke out between Israel and the Arab world. Israel captured the eastern half of Jerusalem in that war, taking full control of the holy city.

The reason all of this matters is because countries almost always put their embassies in capital cities—for example, the US embassy in Turkey is in Ankara, not Istanbul, even though Istanbul’s a bigger city. Moving the US embassy to Jerusalem signifies that America recognizes that Jerusalem is the official capital of Israel. This may make Israel happy, but it’s bound to anger not just Palestinians but people all over the Arab and Muslim world.

Most significantly, no other country in the world has its embassy in Jerusalem. If the US becomes the only one to do so, it could upset the whole balance of power in one of the most explosive regions on Earth.

The order Trump signed on Thursday only puts off the decision about whether to move the embassy for six months. So we’ll be hearing more about this issue next winter.

This article was originally published in January. It has been updated to reflect recent news.