The importance of the fight to keep Jerusalem from being divided is something that many Americans don't understand. As it's often said, almost nobody goes to East Jerusalem anyway, and we're not even supposed to visit the Temple Mount. So why fight so hard against world opinion to keep it?
Here's one very important reason. There is a new trend in the Arab world of "Temple Denial" - denying that there ever was a Jewish Temple in Jerusalem, and correspondingly denying that there ever was a historic Jewish presence in Israel. (See Dore Gold's important book on this.)
The consequences of this are immense. After all, if there was never a historic presence of Jews in Israel, then we really are stealing the entire land from the Arabs. As Rashi on the first verse in the Torah quotes the nations as saying, Listim atem! You stole our land! Since this is what the new generation of Arabs has been taught to believe, then peace is impossible - for, from their perspective, their cause is just.
Now, the presence of the Jewish Temple in Jerusalem is simple historical fact, which is completely accepted in the Western world. So one would expect that Western political leaders would have no reason not to say so. But when Barack Obama gave his Cairo speech a while ago, he made no mention of it. Instead, he spoke only of the horrors of the Holocaust as a justification for Jews to be Israel.
Clearly, Obama felt that it would cause trouble to mention the historical connection between Jews and Israel - that it would be counter-productive to the goal of peace. But this is getting things exactly the wrong way around. Peace will certainly never be achieved if the Arab world believes that we have no right to be here. The Holocaust is not enough; if Jews were never historically in Israel, then why should the Palestinians pay the price for the Holocaust?
Temple Denial, as insane as it is, should not be underestimated. There is an increasing tendency for journalists to write as though the presence of a Jewish Temple is not a matter of historic fact, but is instead the subject of "competing narratives." This is extremely dangerous for the future of Israel.
It's rare to find something that all Jews, of all denominations and sects, can agree on. We don't all agree even on basic things, such as the significance of the State of Israel, the origins of the Torah, or the existence of God. So when there is something that all Jews agree on, despite others in the world who dispute it, it's something special. And all Jews agree that, thousands of years ago, there was a Jewish Temple in Jerusalem.
Yom Yerushalayim is more important now than ever before.