We all know about the army navy games. Often armed services hate each other. I am sure that somewhere, somehow, there has been a war between two armed services in the same country.
I'm not sure if it counts as a "war", but the Battle for Castle Itter on May 5, 1945 was certainly a battle between two service branches of the same country - the Wehrmacht and the Waffen SS. A unique feature of this battle was that the Wehrmacht were allied with a unit from the US Army.
Schloss Itter is a medieval castle in the Austrian North Tyrol village of Itter. During the Second World War it became a sub-unit of the Dachau concentration camp used for VIP prisoners, prominent politicians and military figures that the Nazis wanted to use as bargaining chips. Prisoners there included two former prime ministers of France, Edouard Daladier and Paul Reynaud, as well as Marie-Agnes Cailliau, the elder sister of General Charles de Gaulle.
In May 1945, the German guards at Schloss Itter fled, but the prisoners were trapped in the castle by roaming units of Waffen SS and Gestapo. The prisoners managed to contact a German major, Josef (Sepp) Gangl who had become opposed to the Nazis and was collaborating with the Austrian resistance.
At that time, Gangl only had a couple of dozen loyal troops with him. Under a flag of truce, he made contact with the nearest American unit - from the 23rd Tank Battalion of the US 12th Armoured Division, commanded by Captain Jack Lee.
Lee offered to lead a rescue mission to the castle.
They reached the castle, but at dawn on 5 May, they were attacked by a large force of Waffen SS. The SS succeeded in blowing up the American Sherman tank protecting the castle gate, but were unable to storm the castle. The defending forces held out until a relief force from the 142nd Infantry Regiment arrived, encircling and capturing about 100 Waffen SS troops.
The combined US-German force only suffered a single casualty - Major Gangl was killed by an SS sniper.
The battle is often remembered as the day the US Army joined up with the Wehrmacht to fight the SS.
The definition of war
a state of usually open and declared armed hostile conflict between states or nations
War is between states.
If you have two different service branches fighting, then they are pretty much by definition not from the same country. Might be a mutiny or a coup. Might be a civil war, but in all of those cases, the two forces are serving - or attempting to serve - different states.
There was the Battle of Castle Itter, at the end of WW2, where Wehrmacht and a small number of US forces joined together to fight off a SS division attempting to recapture the Castle Itter. The castle was a prison for high ranking French political prisoners. Presumably, the SS intended to capture the castle to execute the prisoners.
Relief arrived after prisoner and peacetime tennis star Jean Borotra had run on foot from the castle to the main US forces some miles away, to alert them to the precarious position of their advance unit.
Yes, it happened. Cavalry vs infantry. With some casualties. But it was an accident: the Battle of Karánsebes.