The U.S. recently indicted an Irish firm for illegally selling helicopter and F-5 jet fighter parts to Iran between 2005-8. This indictment is a big deal, because smugglers like this keep the Iranian armed forces operational. Nearly 300 American made F-5s were bought by Iran in the 1960s and 1970s, and a few dozen of these are still operational. Many Western nations have become more aggressive in this area of late. Despite the bravado expressed by Iran, about the capabilities of its armed forces, not a lot of modern weaponry and equipment is making its way into the country. So keeping the older stuff in operation is a big deal.
Ever since the U.S. embargo was imposed in 1979 (after Iran broke diplomatic protocol by seizing the American embassy), Iran has sought, with some success, to offer big money to smugglers who can beat the embargo and get needed industrial and military equipment. This is a risky business, and American and European prisons are full of Iranians, and other nationals, who tried, and often failed, to procure forbidden goods. The smuggling operations are currently under more scrutiny, and attack, because of Iran's growing nuclear weapons program. But the Iranians simply offer more money, and more smugglers step up to keep the goodies coming.
The U.S. has gotten more aggressive, and successful, at shutting down Iranian smuggling operations. Not just by bribing the smugglers themselves, but also by getting the cooperation of nations the smugglers operate out of. This has been so successful that most of these smugglers no longer feel safe working out of Arab Persian Gulf nations (especially the United Arab Emirates). As a result, more smugglers are operating out of Malaysia, and the U.S. is trying to shut down that activity. America also monitors the international banking network, seeking signs of smuggler activity, and leaning on the banks involved, to step back.
The smuggling effort has been a mixed success. The Iranian armed forces are poorly equipped, because new tanks, warplanes and ships could not be sneaked in. Thus major weapons acquired in the 1970s are falling apart for want of sufficient replacement parts. Those that are still working, are doing so just barely.