Wehrmacht
Rear-echelon personnel were more often sent to front-line duty wherever possible, especially during the final two years of the war where, inspired by constant propaganda, the oldest and youngest were being recruited and driven by instilled fear and fanaticism to serve on the fronts and, often, to fight to the death, whether judged to be cannon fodder or elite troops.

Prior to World War II, the Wehrmacht strove to remain a purely ethnic German force; as such, minorities within and outside of Germany, such as the Czechs in annexed Czechoslovakia, were exempted from military service after Hitler's takeover in 1938. Foreign volunteers were generally not accepted in the German armed forces prior to 1941. With the invasion of the Soviet Union in 1941, the government's positions changed. German propagandists wanted to present the war not as a purely German concern, but as a multi-national crusade against the so-called Jewish Bolshevism. Hence, the Wehrmacht and the SS began to seek out recruits from occupied and neutral countries across Europe: the Germanic populations of the Netherlands and Norway were recruited largely into the SS, while "non-Germanic" people were recruited into the Wehrmacht. The "voluntary" nature of such recruitment was often dubious, especially in the later years of the war when even Poles living in the Polish Corridor were declared "ethnic Germans" and drafted.
After Germany's defeat in the Battle of Stalingrad, the Wehrmacht also made substantial use of personnel from the Soviet Union, including the Caucasian Muslim Legion, Turkestan Legion, Crimean Tatars, ethnic Ukrainians and Russians, Cossacks, and others who wished to fight against the Soviet regime or who were otherwise induced to join. Between 15,000 and 20,000 anti-communist White émigrés who had left Russia after the Russian Revolution joined the ranks of the Wehrmacht and Waffen-SS, with 1,500 acting as interpreters and more than 10,000 serving in the guard force of the Russian Protective Corps.

Women in the Wehrmacht: In the beginning, women in Nazi Germany were not involved in the Wehrmacht, as Hitler ideologically opposed conscription for women, stating that Germany would "not form any section of women grenade throwers or any corps of women elite snipers." However, with many men going to the front, women were placed in auxiliary positions within the Wehrmacht, called Wehrmachtshelferinnen (lit. 'Female Wehrmacht Helper'), participating in tasks as:

telephone, telegraph and transmission operators,
administrative clerks, typists and messengers,
operators of listening equipment, in anti-aircraft defense, operating projectors for anti-aircraft defense, employees within meteorology services, and auxiliary civil defense personnel
volunteer nurses in military health service, as the German Red Cross or other voluntary organizations.
They were placed under the same authority as (Hiwis), auxiliary personnel of the army (German: Behelfspersonal) and they were assigned to duties within the Reich, and to a lesser extent, in the occupied territories, for example in the general government of occupied Poland, in France, and later in Yugoslavia, in Greece and in Romania.
By 1945, 500,000 women were serving as Wehrmachtshelferinnen, half of whom were volunteers, while the other half performed obligatory services connected to the war effort (German: Kriegshilfsdienst).

Command structure: Legally, the commander-in-chief of the Wehrmacht was Adolf Hitler in his capacity as Germany's head of state, a position he gained after the death of President Paul von Hindenburg in August 1934. With the creation of the Wehrmacht in 1935, Hitler elevated himself to Supreme Commander of the Armed Forces, retaining the position until his suicide on 30 April 1945. The title of Commander-in-Chief was given to the Minister of the Reichswehr Werner von Blomberg, who was simultaneously renamed the Reich Minister of War. Following the Blomberg-Fritsch Affair, Blomberg resigned and Hitler abolished the Ministry of War. As a replacement for the ministry, the Wehrmacht High Command Oberkommando der Wehrmacht (OKW), under Field Marshal Wilhelm Keitel, was put in its place.
Placed under the OKW were the three branch High Commands: Oberkommando des Heeres (OKH), Oberkommando der Marine (OKM), and Oberkommando der Luftwaffe (OKL). The OKW was intended to serve as a joint command and coordinate all military activities, with Hitler at the top. Though many senior officers, such as von Manstein, had advocated for a real tri-service Joint Command, or appointment of a single Joint Chief of Staff, Hitler refused. Even after the defeat at Stalingrad, Hitler refused, stating that Göring as Reichsmarschall and Hitler's deputy, would not submit to someone else or see himself as an equal to other service commanders. However, a more likely reason was Hitler feared it would break his image of having the "Midas touch" concerning military strategy.
With the creation of the OKW, Hitler solidified his control over the Wehrmacht. Showing restraint at the beginning of the war, Hitler also became increasingly involved in military operations at every scale.
Additionally, there was a clear lack of cohesion between the three High Commands and the OKW, as senior generals were unaware of the needs, capabilities and limitations of the other branches. With Hitler serving as Supreme Commander, branch commands were often forced to fight for influence with Hitler. However, influence with Hitler not only came from rank and merit but also who Hitler perceived as loyal, leading to inter-service rivalry, rather than cohesion between his military advisers.

Branches: 

Army: The German Army furthered concepts pioneered during World War I, combining ground (Heer) and air force (Luftwaffe) assets into combined arms teams. Coupled with traditional war fighting methods such as encirclements and the "battle of annihilation", the Wehrmacht managed many lightning quick victories in the first year of World War II, prompting foreign journalists to create a new word for what they witnessed: Blitzkrieg. Germany's immediate military success on the field at the start of the Second World War coincides the favorable beginning they achieved during the First World War, a fact which some attribute to their superior officer corps.
The Heer entered the war with a minority of its formations motorized; infantry remained approximately 90% foot-borne throughout the war, and artillery was primarily horse-drawn. The motorized formations received much attention in the world press in the opening years of the war, and were cited as the reason for the success of the invasions of Poland (September 1939), Denmark and Norway (April 1940), Belgium, France, and Netherlands (May 1940), Yugoslavia and Greece (April 1941) and the early stage of Operation Barbarossa in the Soviet Union (June 1941).
After Hitler declared war on the United States in December 1941, the Axis powers found themselves engaged in campaigns against several major industrial powers while Germany was still in transition to a war economy. German units were then overextended, undersupplied, outmaneuvered, outnumbered and defeated by its enemies in decisive battles during 1941, 1942, and 1943 at the Battle of Moscow, the Siege of Leningrad, Stalingrad, Tunis in North Africa, and the Battle of Kursk.

The German Army was managed through mission-based tactics (rather than order-based tactics) which was intended to give commanders greater freedom to act on events and exploit opportunities. In public opinion, the German Army was, and sometimes still is, seen as a high-tech army. However, such modern equipment, while featured much in propaganda, was often only available in relatively small numbers. Only 40% to 60% of all units in the Eastern Front were motorized, baggage trains often relied on horse-drawn trailers due to poor roads and weather conditions in the Soviet Union, and for the same reasons many soldiers marched on foot or used bicycles as bicycle infantry. As the fortunes of war turned against them, the Germans were in constant retreat from 1943 and onward.: 142 
The Panzer divisions were vital to the German army's early success. In the strategies of the Blitzkrieg, the Wehrmacht combined the mobility of light tanks with airborne assault to quickly progress through weak enemy lines, enabling the German army to quickly take over Poland and France. These tanks were used to break through enemy lines, isolating regiments from the main force so that the infantry behind the tanks could quickly kill or capture the enemy troops.

Air Force: Originally outlawed by the Treaty of Versailles, the Luftwaffe was officially established in 1935, under the leadership of Hermann Göring. First gaining experience in the Spanish Civil War, it was a key element in the early Blitzkrieg campaigns (Poland, France 1940, USSR 1941). The Luftwaffe concentrated production on fighters and (small) tactical bombers, like the Messerschmitt Bf 109 fighter and the Junkers Ju 87 Stuka dive bomber. The planes cooperated closely with the ground forces. Overwhelming numbers of fighters assured air-supremacy, and the bombers would attack command- and supply-lines, depots, and other support targets close to the front.