I don't think people realize just how close both World Wars were to one another. After WWI came to an end, Europe was thrust back into war after only 21 years, while the United States managed to avoid another war for 23 years. Imagine another World War erupting in 1987 then another one erupting in 2010. But to answer your question, YES...thousands..of soldiers, not just Germans had seen combat before the outbreak of WWII. Most of the great Generals, Axis and Allies, fought in both wars. Also any of the professional soldiers who fought in 1919 were still around in 1936 when WWII started heating up.

Notable Axis leaders:

• Adolph Hitler: Dispatch runner on the Western Front in France and Belgium in WWI/ Führer of Nazi Germany in WWII.

• Hasso von Manteuffel: Cavalry officer in WWI/Tank General, given command of the 7th Panzer Grenadier Brigade of the 7th Panzer Division, 5th Panzer Army, Großdeutschland Division, and the 3rd Panzer Army during WWII.

• Karl Doenitz: Lieutenant on U-39 also a POW in WWI/ Achieved the rank of Grand Admiral and Commander in chief of the German Navy in WWII along with being Hitler's successor after his death.

• Heinz Wilhelm Guderian: Signals Officer in WWI/Commanded XIX Corps and was the Chief of Staff of German Army in WWII.

• Erwin Rommel: Fought as a lieutenant in France, Romania and Italy during WWI/ Commander of the 7th Panzer Division, 21st Panzer Division, and the Afrika Korps throughout WWII.

• Fedor von Bock: Served with the 4th Foot Guards Regiment as a battalion commander in WWI/ German field marshal who commanded Army Group North, Army Group B, Army Group Center, and Army Group South throughout WWII.

Notable Allies:

• George S. Patton: Led the U.S. 1st Provisional Tank Brigade in WWI/Commanded the U.S. 2nd Armored Division, U.S. Seventh Army, and the U.S. Third Army in the Mediterranean and European Theaters during World War II.

• Douglas MacArthur: Chief of staff of the 42nd (Rainbow) Division in WWI/5 Star General during the war. Commanded the U.S. Army Forces in the Far East, Supreme Commander of Allied Forces in the Southwest Pacific Area, Supreme Commander for the Allied Powers, and General of the Army during WWII.

• Bernard Montgomery: Junior officer of the Royal Warwickshire Regiment in WWI/ Commanded the British Eighth Army in North Africa, the 21st Army Group, and commander of all Allied ground forces during Operation Overlord during WWII.

• Roy Stanley Geiger: Commanded a squadron of the First Marine Aviation Force WWI/ 4 Star General, commanding the 1st Marine Aircraft Wing, Fleet Marine Force, I Amphibious Corps, and the Fleet Marine Force, Pacific during WWII.

• Mark W. Clark: Served in France with the U.S. 11th Infantry, part of the 5th Infantry Division in WWI/ Commanded US Fifth Army, and the Fifteenth Army Group in WWII.

Career soldiers of both the Axis and Allies armies fought in combatant roles during both wars. Almost all of the top names that we associate with WWII history served in both wars in some form or another. Senior officers were almost all WWI veterans on both sides.

I'm assuming if one took the time to research the percentage or quantity of Officers service records from the first World War due to continuation of service after 1921, you could start to understand how many junior officers carried over from WWI to WWII or rejoined between 1939-1941.

Maybe one of the historians could help me out on this but I couldn't find hard numbers regarding WWI veterans who went on to fight in WWII but I found some information regarding the Korean War that puts some numbers into context.

Out of the 12,209,238 million men and women who served in the United States Military during WWII, 997,000 went on to fight in the Korean War, and 347,000 of that number also fought in the Vietnam War. 291,000 U.S. Military personal served in all three wars. (WWII, Korea, and Vietnam)