Ibarreche took part in the Spanish Civil War and he joined the Nationalist Army at the end of the Biscay campaign.

He completed his pilot training in January 1938 from where he was posted direct to Group 2-G-3. This unit was equipped with the Fiat CR.32.

During a second sortie of the day on 19 June, ten Fiats of 2-G-3 took off at 18:00 led by capitán Ángel Salas, to escort Ju 52/3ms bombing La Puebla de Valverde. They encountered a formation of 18 Chatos, which they chased as far as Alcublas, where nine Ratas joined the fray. Capitán Salas dived over the Chatos to attack, but was unable to fire his guns because a leak had emptied his compressed air bottle. Despite this, he continued to make dummy attacks, being hit five times by the Ratas, one bullet puncturing the coolant radiator. Teniente Julio Salvador (CO 1-E-3) enjoyed better luck, bouncing two Chatos from above near La Puebla, one of which exploded in the air, the other following in flames. It seems that these come from the 3^a Escuadrilla and both pilots survived with wounds. Salvador then had to retire with an overheating engine. Teniente Miguel García Pardo (2-E-3) effectively removed a Rata from the tail of alférez Arístides García López Rengel’s (1-E-3) Fiat by shooting it down. Teniente de Hemricourt (2-E-3) downed a Chato near Alcublas, then a Rata to the north of Villar del Arzobispo, and saw a Chato turn somersault as it attempted to land at its aerodrome. Teniente Ibarreche fired at a Chato close to the ground, and this separated from the rest of the formation and fell near to Hiruerela. Alférez Joaquín Ansaldo Vejarano (2-E-3) had to return to base when an engine cowling parted company from his Fiat.
No CR.32s were lost in this combat.

In the morning on 2 September, a formation of nine R-Zs, escorted by a similar number of I-15s, were attacked by 18 Spanish-flown CR.32s near Monterrubio de la Serena as they returned from a bombing mission. Minutes later nine SBs from 4^a Escuadrilla of Grupo N^o 24, escorted at a distance by 11 I-16s Type 10, hove into view too. The CR.32s concentrated on the Tupolev bombers and their monoplane fighter escorts. In a single attack, capitán Ángel Salas (2-G-3) destroyed three Katiuskas, and then capped this by damaging the I-16 Type 10 flown by the leader of the 1^a Escuadrilla Grupo de Moscas N^o 21 that was escorting them. This pilot, teniente José Redondo Martín (the son of the former mayor of Madrid, Cayetano Redondo Aceña), took to his parachute and was guarded by Salas until he had almost reached the ground. Salas, before flying away, raised an arm in the Fascist salute, and Redondo (I-16 ‘CM-214’) responded in a similar manner with a clenched fist.
Salas noted in his logbook:

“After 50 minutes on patrol I spotted nine “Martin bombers” that were a little higher than us heading in the direction of our lines. I gained height as I flew towards them, cutting off their escape route. As they returned after bombing Monterrubio, I attacked the bomber on the left in the first section, setting his left engine on fire. I then hit the left engine of the leader’s aircraft, setting it on fire too, and the third bomber suffered the same fate. Then I fired straight up at the leader of the second flight, and his aircraft started trailing white smoke. It was then that I noticed some Ratas, so I attacked one. After a long fight it also started to trail smoke, and the pilot took to his parachute. He landed close to his aircraft, north of Belalcázar. During his descent I saluted him and he saluted me back.”

“The part played by the group under your command in today's operation has filled me with pride and it gives me great satisfaction to have people under my command who know how to make the best use of prevailing circumstances. It gives me great pleasure to congratulate you personally, and the whole of your unit, to whom I hope you will convey this message.”

On 20 September, a series of daily battles began, culminating in fierce combats on 2 and 3 October. Groups 2-G-3 and 3-G-3 fought jointly in all these battles, achieving a total of twenty-five victories (17 by 2-G-3). Of these Ibarreche claimed two.

After the end of the war, he was decorated with the Cruz Militar (the Military Medal) and promoted for his services during the war. The medal was however not presented to him until the day of Our Lady of Lareto in 1941, by which time he was involved in a new war!

Ibarreche ended the Civil War with 7 biplane victories of which three were I-16s and four were I-15s. He also claimed 2 shared victories.

He later volunteered for combat in Russia and was part of the 1^a Escuadrilla Azul, (Escuadrilla Expedicionaria) which was formed with 17 pilots with Salas as the Commander. Some of them were veterans of the Spanish Civil War and had a combined score of 79 aerial victories.

The unit crossed into France on 24 July 1941, later arriving in Germany in late part of the month.

They were assigned to training at the Jagdfliegerrschule 1 at Werneuchen near Berlin.

After completing their training on Bf 109 Es, the 1^a Escuadrilla left for Russia on 26 September.
At first, the Spanish pilots were worried that they would arrive late to see any action, since they were kept informed of the progress of the German advance towards Moscow.

Azul Escuadrilla is welcomed on the East Front at Moschna airfield in September 1941.

The 1^a Escuadrilla received a total of 12 Bf 109 E-4s and –7s and they were assigned to JG 27, VIII FliegerKorps of LuftFlotte 2. They were known as the “15 Spanische Staffel”, and they started to fly their first combat operations on 2 October. Escorting German bombers and reconnaissance aircraft, they operated from 12 different airfields.
Already on the first day, 2 October, they suffered their first pilot causality, when Luis Alcocer (Bf 109 E-7 WNr. 1390) was shot down and killed in air combat with MiG-1s, I-16s and Il-4s 6 kilometres north of Moschna.

On 13 October, Captain Carlos Bayo Alessandri claimed an I-18 while Lieutenant Ibarreche claimed a biplane.

Ibarreche claimed and SB-3 on 14 October.

The 1^a Escuadrilla Azul received the orders to return to Spain on 6 January 1942.
During their time in Russia the unit flew a total of 460 missions, were engaged in 94 aerial combats and destroyed 10 enemy aircraft plus four on the ground, for the loss of 5 pilots (one KIFA, three MIA and one KIA) plus one wounded in action.
Ibarreche claimed 1 victory during his time in Russia.

Esteban Ibarreche ended the war with 7 biplane victories and a total of 9. All of the biplane victories were claimed during the Spanish Civil War.

Claims:

Kill no.	Number	Date	Time	Type	Result	Plane type	Serial no.	Locality	Unit
	1938
1	19/06/38	18:00-	1	I-15	Destroyed	Fiat CR.32		Hiruerela area	2-G-3
2	02/09/38	morning	1	I-16	Destroyed	Fiat CR.32		Zújar area	2-G-3
3	??/??/38		1	I-16	Destroyed	Fiat CR.32			2-G-3
4	??/??/38		1	I-16	Destroyed	Fiat CR.32			2-G-3
5	??/??/38		1	I-15	Destroyed	Fiat CR.32			2-G-3
6	??/??/38		1	I-15	Destroyed	Fiat CR.32			2-G-3
7	??/??/38		1	I-15	Destroyed	Fiat CR.32			2-G-3
	??/??/3?		1	Enemy aircraft	Shared destroyed	Fiat CR.32
	??/??/3?		1	I-15	Shared destroyed	Fiat CR.32
	1941
8	13/10/41		1	Biplane	Destroyed	Bf 109 E		Russia	1a Escuadrilla Azul
9	14/10/41		1	SB-3	Destroyed	Bf 109 E		Russia	1a Escuadrilla Azul

Biplane victories: 7 and 2 shared destroyed.
TOTAL: 9 and 2 shared destroyed.

Sources:
Air War over Spain - Jesus Salas Larrazabal, 1974 Ian Allan Ltd, Shepperton, Surrey, ISBN 0-7110-0521-4
Angel Salas Larrazabal - A Fighter Ace in Two Wars (WWII Ace Stories) - Santiago A. Flores, 2001
Deutsche Luftwaffe Losses & Claims -series - Michael Balss
Fiat CR.32 Aces of the Spanish Civil War - Alfredo Logoluso, 2010 Osprey Publishing, Oxford, ISBN 978-1-84603-983-6
Några leva än - F. G. Tinker, 1939 T. V. Scheutz Bokförlag AB, Stockholm
The Facile Fiat...Rosatelli's "Italian Fighter" - Air Enthusiast/Twenty-Two
The Legion Condor - Karl Ries and Hans Ring, 1992 Schiffer Publishing, ISBN 0-88740-339-5
Additional information kindly provided by Christer Bergström and Tom Semenza.