Why did Iran not come to the aid of Iraq during the Persian Gulf War?

Iran did not come to the aid of Iraq during the Persian Gulf War because the two countries were enemies at that time.  There were at least two reasons why they were enemies.

First, Iran and Iraq were enemies because they had fought a very bloody war in the recent past.  The Persian Gulf War happened in late 1990 and early 1991.  At that point, it had only been a couple of years since Iran and Iraq had ended their war.  The two countries fought a war that lasted from 1980 to 1988.  It would be very odd if a country would, only two years after ending a war, come to the aid of the country it had been fighting.

Second, Iraq and Iran had very different governments at this time.  Iraq’s government was a secular military dictatorship led by Saddam Hussein.  It was made up mainly of Sunni Muslims.  Iran’s government was a theocracy, made up mainly of Shia Muslims.  The two countries were divided both by their religious beliefs and by their type of government.  Even if the two had not just fought a war, it is not clear that Iran would have been eager to help Iraq in this war.

In the Persian Gulf War, Iran would not have had any reason to help Iraq as the two were enemy countries that had very recently fought a brutal war against one another.