In science it is said that every action has an equal and opposite reaction. In that vein, Iran and Iraq largely cancelled out each other's power at the time, by each keeping the other in check.
While the balance of power between Iraq and Iran prior to the US invasion was arguably incident free under the watchful eye of the US, when Iraq was invaded two notable things happened. The ''standoff' and push back by Iraq against Iran was eliminated and the overseeing of that 'condition' was nullified as the US now had its hands full fighting in Iraq and Afghanistan, and was not then able to police the region in any meaningful sense.
The US invasion of Iraq essentially removed Iraq from being an opposing force against Iran, thereby providing the freedom for Iran to pursue its many goals, on of which was the realization of Nuclear independence.
When Bush started the push to invade Iraq, Iran at that time had about 20 barely working centrifuges, when Obama was sworn in, by 2008, Iran had thousands of operable centrifuges ... and was by then also sitting on some 2000 lbs of uranium that they had made themselves!... 'By the time Bush left office in January 2009, Iran had just under 4,000 working centrifuges and an additional 1,600 installed. '...
Defense One...'Zero is a fantasy, and you can blame President George W. Bush’s administration for that. It may have been possible to convince Iran to dismantle all its centrifuges when it had only a few dozen in 2003 and first offered to talk to the U.S.. Or in 2005, when it had a few hundred and was in talks with the European Union. But the Bush administration spurned any deal.'
In short the Iranian lion cub had eight unimpeded years of growth and had become a different animal, one that was now full grown and had a deafening Nuclear roar.
Any chance of curbing or eliminating the Nuclear progress Iran had enjoyed under the Republican minions' watch, was now a folly of unimaginative conservative thinking. They had created a monster which had outgrown its cage and shackles.
The incoming Obama administration was by then saddled with wars in Iraq and Afghanistan coupled with a deep recession that was fast becoming a great global depression, while combating internal sabotage by the republicans in congress at every turn... all these issues effectively tied the hands of the US government.
Consequently Iran was able to build on its arsenal of 5,600 centrifuges in 2008, increasing that number to over 19,000 in 2015.
The Republican cries over the recent nuclear deal with Iran is particularly concerning as the republicans are still of the mistaken mindset that Iran is just a fly that can be swatted and not be negotiated with, as they were in the early 2000's. Republicans decry the deal as if it was made by uninformed people from insignificant countries.
Republicans are still not aware of Iran's stronger stature in Nuclear development. Their clueless talking points from all corners confirm that. Iran has now attained the power it needed to become a big player in the Nuclear race and must now be negotiated with, according to its standing.
Most troubling is the upcoming 2016 elections. The republican roar of war rhetoric is being heard again. The republican clown car has turned into a bus, with each candidate flinging their own misguided versions of vague foreign policy at their base... much like the caged monkeys flinging their dung in the zoo. The world is indeed facing perilous times... and the Republican party is just warming up... again.