In early September, the US accused Iraq of allowing Iran to ship arms to Syria across their airspace. Iraq demanded evidence, none was ever provided, but after weeks of browbeating, Iraq finally gave in and promised to start randomly searching planes.
Which was in retrospect probably not a great move. The planes were searched, nothing was found, and the Obama Administration is once again mad at Iraq, despite them having already given in, for not delivering the evidence they were hoping for.
Prime Minister Nouri al-Maliki concedes that there’s basically nothing for it at this point, since they don’t have the capacity to even theoretically search every single Iranian civilian and cargo flight going across the country.
As close allies, Syria and Iran are also extremely major trading partners, and naturally the bulk of that trade flies over Iraqi airspace. So long as that trade continues, the US is going to assume ulterior motives, and Iraq, caught in the middle, will be the scapegoat for failing to confirm US theories of what is happening.