When the United States joined World War II, in 1941, von Ripper attempted to join the army as a soldier, but was initially rejected due to his health.
Meanwhile, he painted propaganda posters intended for distribution abroad for the Office of War Information.

Eventually, on September 5, 1942, he was admitted to the United States Army for 'limited service only' due to his wounds, and initially served as a hospital laboratory technician.
On the formation of the Corps of Engineers War Art Unit, he transferred to that unit and in 1943 sent to North Africa as an Artist Correspondent alongside Mitchell Siporin.

In 1943, he became a United States citizen.

He worked alongside Pulitzer Prize-winning journalist Ernie Pyle, who in his 1944 book Brave Men wrote:
One of the most fabulous characters in that war theater was Lieutenant Rudolf Charles von Ripper.
He was so fabulous, a man might have been justified in thinking him a phony until he got to know him. ...
He was no phony.
Von Ripper was the kind they write books about.

— Brave Men, by Ernie Pyle, 1944
The Army Artist Unit was dissolved in May 1943, and von Ripper transferred to the Intelligence Section to interrogate prisoners.
 He was assigned to the 34th Infantry Division, with whom he took part in the Invasion of Italy.

In Italy, he served as acting intelligence officer of the 2nd Battalion of the 168th Infantry Regiment.
 He also led patrols against Nazi positions, either with squads of soldiers or alone.
For actions in these sorties he was awarded a Silver Star and oak leaf cluster, and on December 12 1943 was promoted to second lieutenant, which fellow artist Sergeant Mitchell Siporin described as 'a battlefield appointment about which much should be said'.
He also received a Purple Heart with oak leaf cluster and a Division Citation.

Von Ripper and one of his men advanced in the face of intense enemy fire and killed two and wounded three of the enemy, took 11 prisoners and captured numerous enemy weapons
— Citation for Oak leaf cluster to a Silver Star, quoted in Von Ripper: Painter with a Gun by Peter Carlson
In a later sortie, he was ambushed and wounded in his right hand, left leg and face by machine gun bullets.
He was taken to Naples to recover, where he continued to work on paintings based on his front line sketches.
He became sufficiently known for his audacity when leading patrols that his division forbade him going on patrol without specific permission.

In December 1943, his art was included in The Army at War, a War Department exhibition of combat art.
Writing in the catalog for the exhibition, von Ripper wrote: 
A soldier-artist is a painter with a gun, a man to fight at times and to paint at other times.
And in that he is very lucky: He can divert his effort from destruction, from killing, which is the soldier’s job, to creative work, to build, make new things.

— Rudolph von Ripper, quoted in The Army at War, War Finance Department 1943
In February 1944 he returned to the front lines and was involved in the Battle of Monte Cassino.

von Ripper's exploits drew the attention of Office of Strategic Services (OSS) director William J. Donovan, and he was recruited for its Secret Intelligence Branch.
He parachuted into Austria early in 1945 to organize resistance and inform the OSS about the situation.
After the war ended, von Ripper was involved in finding Gestapo and Nazi officials hiding in Austria.
When the United States joined World War II, in 1941, von Ripper attempted to join the army as a soldier, but was initially rejected due to his health. Meanwhile, he painted propaganda posters intended for distribution abroad for the Office of War Information. 
Eventually, on September 5, 1942, he was admitted to the United States Army for 'limited service only' due to his wounds, and initially served as a hospital laboratory technician. On the formation of the Corps of Engineers War Art Unit, he transferred to that unit and in 1943 sent to North Africa as an Artist Correspondent alongside Mitchell Siporin.
In 1943, he became a United States citizen.
He worked alongside Pulitzer Prize-winning journalist Ernie Pyle, who in his 1944 book Brave Men wrote:
One of the most fabulous characters in that war theater was Lieutenant Rudolf Charles von Ripper. He was so fabulous, a man might have been justified in thinking him a phony until he got to know him. ... He was no phony. Von Ripper was the kind they write books about.
— Brave Men, by Ernie Pyle, 1944
The Army Artist Unit was dissolved in May 1943, and von Ripper transferred to the Intelligence Section to interrogate prisoners.  He was assigned to the 34th Infantry Division, with whom he took part in the Invasion of Italy.
In Italy, he served as acting intelligence officer of the 2nd Battalion of the 168th Infantry Regiment.  He also led patrols against Nazi positions, either with squads of soldiers or alone. For actions in these sorties he was awarded a Silver Star and oak leaf cluster, and on December 12 1943 was promoted to second lieutenant, which fellow artist Sergeant Mitchell Siporin described as 'a battlefield appointment about which much should be said'. He also received a Purple Heart with oak leaf cluster and a Division Citation.
Von Ripper and one of his men advanced in the face of intense enemy fire and killed two and wounded three of the enemy, took 11 prisoners and captured numerous enemy weapons
— Citation for Oak leaf cluster to a Silver Star, quoted in Von Ripper: Painter with a Gun by Peter Carlson
In a later sortie, he was ambushed and wounded in his right hand, left leg and face by machine gun bullets. He was taken to Naples to recover, where he continued to work on paintings based on his front line sketches. He became sufficiently known for his audacity when leading patrols that his division forbade him going on patrol without specific permission.
In December 1943, his art was included in The Army at War, a War Department exhibition of combat art. Writing in the catalog for the exhibition, von Ripper wrote: 
A soldier-artist is a painter with a gun, a man to fight at times and to paint at other times. And in that he is very lucky: He can divert his effort from destruction, from killing, which is the soldier’s job, to creative work, to build, make new things.
— Rudolph von Ripper, quoted in The Army at War, War Finance Department 1943
In February 1944 he returned to the front lines and was involved in the Battle of Monte Cassino.
von Ripper's exploits drew the attention of Office of Strategic Services (OSS) director William J. Donovan, and he was recruited for its Secret Intelligence Branch. He parachuted into Austria early in 1945 to organize resistance and inform the OSS about the situation. After the war ended, von Ripper was involved in finding Gestapo and Nazi officials hiding in Austria.
Rudolph von Ripper