Rouhani´s three-day visit comes as Iraq has been under pressure from Washington to limit ties with its neighbour, particularly after the United States withdrew from the Iran nuclear deal and hit Tehran with sanctions.
“Iran-Iraq relations are special,” Rouhani said at Tehran´s Mehrabad airport, state television reported.
“Whenever the people of the region had a problem and asked Iran for help, the people and government of Iran rushed to their help with all their power,” he added.
Iran has close but complicated ties with Iraq, with significant influence among its Shiite political groups.
The two countries fought a bloody war from 1980 to 1988 and Tehran´s influence in Baghdad grew after the 2003 US-led invasion of Iraq toppled the government of Saddam Hussein.
Iran was the first country to respond to Iraqi calls for help after Islamic State group jihadists captured Mosul in 2014 and threatened to overrun Baghdad and Kirkuk.
Tehran dispatched “military advisors” and equipment overnight along with the famous Revolutionary Guards elite Qods Force commander Qasem Soleimani to prevent IS jihadists from approaching its western borders.
Following the defeat of IS in Iraq, Iran is trying to position itself to gain a prime role in rebuilding the war-ravaged country.
“Iran-Iraq relations are not comparable with (Iraq´s relations) with an aggressor country like America,” Rouhani said.
“America is despised in the region. The bombs that the Americans dropped on Iraqis, Syrian people and other countries cannot be forgotten and,” he said.
Rouhani´s visit to Iraq will be the first since he became president in 2013. He is set to meet Iraqi Prime Minister Adel Abdel Mahdi, President Barham Saleh and the country´s chief Shiite cleric Grand Ayatollah Ali Sistani, according to the Iranian government´s website.
Iran´s Foreign Minister Javad Zarif arrived Sunday in Baghdad to prepare for the visit.