Found in the march 2 magazine "Le Point" (an interview with Fukuyama):

Didn't America commit a major mistake by letting Iran fill up the strategic void left by Iraq?

One of the basis strategic principles is to avoid a country becoming a dominant power in a given region.
And one of the consequences of the Iraq war is to have let Iraq open to a strong Iranian political influence.
More, while one of the war's objectives was to make countries respect the non proliferation treaty, it seems it had the opposite effect persuading Iran to accelerate its nuclear program in order to dissuade the US from attacking it.