Probably one of the earliest of the 9/11 conspiracy theories to come about is the belief that the 9/11 attacks were committed by the government of Israel as part of some plot to cause the United States to attack the enemies of the nation of Israel.
This conspiracy theory is base on the belief that no Jewish people were killed in the 9/11 attacks, and that all the Jewish people who worked at the World Trade Center site received a phone call that day before the attacks occurred telling them that they needed to stay home.
The first part in which this conspiracy theory is based on, that no Jewish people were killed in the attacks, is completely untrue. In fact it is estimated that somewhere between 270 and 400 Jewish people were killed in the attacks, including five Israeli citizens.
The second part of this conspiracy theory, that all the Jewish people who worked at the World Trade Center site were given a call telling them not to go into work, is obviously not true because there were Jewish people who were killed in the attacks.
Also, not one Jewish person who did work at the World Trade Center site that was not killed that day has ever claimed that they received a phone call telling them to stay home. Considering how many Jewish people probably did work at the World Trade Center site it would be illogical to the point of being impossible that all of them would keep quiet about something like this.
Besides all of this the nation of Israel has no real reason to ever attack the United States.
The United States is Israel's largest and most supportive ally, and President George W. Bush supported Israel more then any other United States president ever. It would make no sense what so ever for Israel to ever attack the United States, and risk losing it's largest ally if ever found out, and seriously risk going to war with the most powerful country on the planet.
The reality is that this conspiracy theory is not based on any evidence what so ever, and by all appearances appears to be nothing more then a complete and total lie created for some sort of anti-Israeli and anti-Semitic propaganda and dis-information.
The only people who claim to believe in this conspiracy theory are usually either white supremacists, neo-nazis, radical and fundamentalist Muslims, and people who generally have strong anti-Semitic beliefs, and are in general more willing to believe in these types of conspiracy theories in the first place, and use such beliefs to continue their "justification" of their bigotry and hatred towards the Jewish people and the nation of Israel.
This conspiracy theory is nothing more then a long line of made up anti-Semitic propaganda, and has no justification what so ever of even existing in the first place.