The Iranian population is 90% Shia Muslim. Iran is 2/3rd Shia and 30% Sunni. The divide between the Shia and Sunni is similar to Protestants and Catholics in the Christian faith. Saddam Hussein was a Sunni who effectively ruled Iraq for and with the minority third of the population, often repressing and persecuting elements of the Shia population during his time in power.
He declared was on Iran. When the dust cleared after the 2nd Gulf War the Shia became the majority power in the Iraqi Parliament and have reasonable relations, jointly collaborated in the fight against ISIS and Iran is Iraq's biggest trading partner. The Iraqi government also has fostered reasonable relations with the US and has been walking a tightrope between the two powers.
The airstrikes against Shia militia late last year and the assassination of Suleimani. both on Iraqi soil has shaken the rope. The first sign of how the Iraqi's have reacted is their request for all US troops to leave Iraqi soil and the shutting down of the multi-nation task force established to remove the ISIS threat.
All balls up in the air but there are a number on here who think that Trump knows exactly what he is doing and will make the wisest decisions at the right time so I'm probably being overly negative.