The British knew what was up, and they still had a few hundred troops in Iraq. By treaty, Britain had the right to move troops through Iraq, and maintain two airbases (outside Baghdad and Basra in the south, mainly for training). Without saying anything to the Iraqis, the British began moving the 10 Indian Infantry division from India to Basra. The first brigade of the division arrived in Basra on April 18th. On April 27th, the Iraqis broke their treaty by demanding that the British land no more troops until the brigade already in Basra had left Iraq. The British told the Iraqis that they would ignore that request. Both sides knew this meant war. The British took over Basra, thus protecting their airbase outside the city. They also flew 400 troops to the Habbaniya airbase outside Baghdad, as they knew that the Iraqis were already moving troops to seize the base.
The British had a battalion of Assyrian (Iraqi Christians) militia, plus a company of Kurds and Arabs, guarding Habbaniya, and sent additional troops to show the Assyrians that they would be supported. The airbase had about 90 aircraft, but nearly all were biplane trainers. The base was used mainly for training pilots. But the pilots were of good quality, particularly the instructors. The Iraqi air force was also equipped mainly with a 60 aircraft, mostly biplanes from three different countries, and the Iraqi pilots were not very competent.
The Iraqi army consisted of four infantry divisions, one mechanized brigade (a battalion with 16 light tanks and fourteen armored cars, plus two battalions of infantry in trucks). Many of the officers and NCOs had served in the Turkish army. Men from Baghdad had served as Turkish civil servants, army officers and NCOs for centuries. But these men were not of the same quality as the Turks. Indeed, the Turks considered the Baghdadis corrupt, effete and unreliable. But they were smart, and they could count. On April 30th, the Iraqis moved 50 pieces of artillery and 9,000 troops to Habbaniya, as well as several times that number (no one got an exact count) of tribal militiamen. The tribal fighters were tough, but undisciplined and poorly led. The Iraqis did the math and decided that Habbaniya would soon be theirs.
The Iraqis occupied the high ground about a thousand yards from Habbaniya, and warned the British that they would open fire with artillery if any British aircraft took off. At dawn on May 2nd, the British aircraft took off anyway, and attacked the Iraqi troops. Over the next five days, the British aircraft launched 584 sorties, dropped 45 tons of bombs and fired over 100,000 rounds of machine-gun ammunition. The Iraqi artillery did some damage, but Iraqi infantry and tribesmen began to flee on the 6th. British troops also launched raids from the base, and by May7th, the only Iraqi troops to be found were fleeing from the British warplanes.
Meanwhile, the rest of the 10th Indian division landed at Basra on May 6th and began advancing on Baghdad. At the same time, the British organized a mobile brigade ("Habforce") and a battalion sized force (KingCol) in Palestine and sent them off towards Baghdad. Led by several hundred scouts from the Arab Legion, Habforce entered Iraq on May 9th, followed by KingCol on the 15th.
The Iraqis now knew they were in big trouble. Even though they outnumbered the attacking British by more than three to one, the British troops were far better trained and led. And the British controlled the air. So the Iraqis looked for stronger allies, and contacted the Germans. Moving quickly, the Germans began flying warplanes from Greece, via Vichy controlled Syria, into northern Iraq. Two German He-111 bombers hit Habbaniya on May 10th. A squadron of He-111s (about a dozen aircraft) were flying into Mosul, as well as a squadron of two engine fighters (Me-110s) and some captured French Dewontine fighters. The Italians sent 12 biplane fighters. The British responded by flying in some Hurricane fighters in from Palestine. On May 13th, the German force began to fly more bombing missions out of Mosul. But the German air operations were limited by a lack of aviation gas at Mosul. And they were unable to fly in more fuel from Syria because British bombers from Palestine were attacking the Syrian air bases.
Faced with an unreliable supply line, a fuel shortage, and British troops advancing on Baghdad, the Germans withdrew their aircraft. Habforce reached Habbaniya on May 17th. On May 23rd, the British force in Habbaniya moved out, crossed the Euphrates river and moved towards Baghdad. The Iraqi resistance got weaker and weaker as they neared the city. On May 30th, the Iraqi resistance ended. The Iraqi leader, Rashid Ali, fled to Iran, and thence made his way to Germany. There he spent the war making anti-Semitic and anti-British radio broadcasts to the Arabs in the Middle East. When Germany fell in 1945, Rashid Ali escaped once more and found refuge in Saudi Arabia. He remained there until 1958, when another army coup killed most of the royal family and Rashid Ali was invited to return home.
Although outnumbered more than three to one, and facing superior German air power, the British conquered Iraq in four weeks.
The Last Time (1941) Iraq Was Invaded. During the 1920s, Britain cobbled together bits of the defunct Ottoman empire and created Iraq. An Arabian prince, driven into exile by the al Saud family, was made king, to insure that Iraq remained pro-British. But when World War II began in 1939, Arab nationalists became pro-German. The anti-Semitism of the Nazis appealed to Arabs, who resented the way Britain was deciding how new Arab nations would be set up, and for supporting continued Jewish settlement in Palestine. As Greece fell to the Germans and the Afrika Korps was advancing across North Africa towards Egypt, pro-German Iraqi army officers staged a coup in early April, 1941. The new Iraqi government didn't declare war on Britain, they just wanted to be on the right side when the Germans won the war.