They welcomed the invasion; Saddam Hussein had been a mortal enemy of Iran ever since the 1980-1989 Iran-Iraq War.
While nowhere near as advanced as China’s various sea mines, ballistic and cruise missiles, submarines, cyber weapons and C2 and C4ISR systems, Iranian A2/AD still packs quite a punch. Iraq was Iran's primary threat after the collapse of the Soviet Union because it was the only direction from which an attack might come.A pro-Iranian or even neutral Iraq would guarantee Iranian national security. The Iranians anticipated this, supporting pro-Iranian elements among the Shia prior to 2003 and shaping them into significant militias after 2003.In 1979, the destinies of Iran and Afghanistan were irrevocably altered.A revolution overthrew the monarchy in Iran, and the Soviet Union invaded Afghanistan. Iran was also concerned about the spread of Saudi "Wahhabism," which Ayatollah Khomeini called, "America's Islam."In the beginning, Iran's Afghan policy was Shiite-centric.