Iran's shah was a big ally of the united states

You might be surprised, because the reason why the USA didn’t help the Shah of Iran, was because that USA, and the West in general feared Iran and it’s potential
Before we go into details, we need to take a look back in history. Mohammad Reza Shah gained control over Iran, in 1953, after the deposal of the democratic elected prime minister, Mossadegh.
The Shah gained control with the help of CIA, and was used as a puppet for the western nations to exploit Iran’s immense oil wealth.
During that time, it was the British that owned all of Iran’s oil fields, and thus it was actually only the British that profited from the sale of the Iranian oil: Iran didn’t earn a single dollar.
But during the 1970’s, the Shah’s behavoir towards the West began to change, and the US took notice. He was no longer an obiedant lap dog, but he wanted to oust Western influence in Iran.
You see, the Shah, Mohammad Reza Pahlavi, was obsessed with modernizing Iran rapidly, and making Iran a superpower on it’s own
This intensified in the 1970s, and resulted in:
Iran actually had the 5th largest military in the world at that time. The Shah spend most of Iran’s oil wealth on buying the newest military technology from the USA, France and the UK. At it’s highest point, more than 14% of Iran’s GDP was spent on the military.
Iran also started to project it’s influence in the broader Middle Eastern region. For example Iran send hundreds of special forces soldiers, during the 1972 rebellion in Oman, to help Oman’s Sultan.
The Shah was also very confident in himself, and told things like this to British interviewers:
The blue-eyed people (ie. the Western people) need to wake up from their complacency, which they put themselves by taking to many sleeping pills
- Mohammad Reza Shah, 1973
He was indeed a savage, but it didn’t exactly please the American leaders.
In another interview, the Shah also called the British people “lazy” and told them to work harder, like the Iranian people did.
The Shah told multiple times, that his goal was that Iran would regain it’s former glory, that it had under the Persian Empire, and become a superpower once again. This was a huge warning sign for the USA, and they feared that their power and influence in the Middle East would soon be rivaled.
The Shah was also capable of manipulating the oil prices and put them at a high price, since Iran was a leading nation in the OPEC, and the largest oil producing state at that time. This frustrated the US, as they liked to gain access to constant cheap oil prices.
The Shah was even starting to build multiple nuclear plants in Iran, with help from European firms, as the Shah wished to erase Iran’s energy dependance on oil and natural gas.
The US simply wished to replace the Shah with another, weaker, and more insignificant puppet leader, and mistakenly thought that Ayatollah Khomeini would fit the bill.
He didn’t however, and this would go down as one of the greatest failures in the US foreign policy.
Sources:
Iran–United States relations - Wikipedia
How the Shah Entangled America
64 Years Later, CIA Finally Releases Details of Iranian Coup
The key moments in the long history of U.S.-Iran tensions