I'd recommend you to this thread [here] (https://www.reddit.com/r/AskHistorians/comments/3ywuud/what_did_the_wehrmacht_get_wrong_in_ww2/). Suffice to say, German military prowess in the Second World War, while not undeserved, is heavily exaggerated.

with the most focus on pre-WWI, where warfare had changed as a result of the railway systems and machine guns (to a point where it seemed as if WWI was a war fought with weapons that even the most experienced veterans didn't know how to properly utilize).

Railways certainly revolutionized warfare prior to WWI, but it was developments in artillery, not the machine gun, that were to have the greatest effect on combat in WWI. 60-70% of casualties on the Western Front, if not in the War as a whole, were inflicted by artillery. I have an answer on pre-war tactics [here] (https://www.reddit.com/r/AskHistorians/comments/3rtu5m/what_kind_of_tactics_did_world_war_1_infantry_use/).

Was the effectiveness of the Nazi army in the West a big result of their air force capabilities, or was it due to other nations not knowing/correctly acting in regards to the Nazi advances? (If you have any input on Nazi military tactics, strategy, or any interesting tidbits about the Nazi military on all fronts I would love to hear them!)

As my answers [here] (https://www.reddit.com/r/AskHistorians/comments/3sc0tv/french_in_world_war_2/), [here] (https://www.reddit.com/r/AskHistorians/comments/3vfiln/german_military_wwii/), and [here] (https://www.reddit.com/r/AskHistorians/comments/3t5288/why_was_there_such_a_quick_decline_in_the/cx3v9xy) should demonstrate, there were many reasons for the stunning successes achieved by the Germans between September, 1939 and June, 1941. In the case of Poland, the Poles were out-gunned, out-manned, and without any support from their allies. They were attacked on a broad front, and in the end were under attack from two sides, by two great powers. Denmark fell easily, but the German Navy was mauled around Norway, and the Germans managed to fight the Franco-British forces there to a stand still, before the invasion of the Low Countries obliged them to withdraw their troops. Yugoslavia was attacked from all sides by crushingly superior German, Italian and Bulgarian forces, and for the Greeks and 'W' Force it was the same, although Crete was a close run thing at the start.

The one time before the Invasion of the Soviet Union that the Wehrmacht faced equal opponents was the Invasion of France and the Low Countries. Between the French and the BEF, the Germans faced an opponent whose armies were more motorized/mechanized than theirs, had more and better armed/armoured tanks than theirs, and equal if not slightly greater aircraft in quantity and quality. The Maginot Line blocked a German offensive via the route of 1870, while an attack through Belgium via the route of 1914 meant going through the Dutch and the Belgians first, by which time the British and French would be dug-in behind the Dyle and Meuse Rivers, and in the Gembloux Gap. When German armoured forces encountered French, Belgian and British armour around Gembloux, Hannut, and later at Arras, they generally did not have their way.

What counted was that the bulk of Germany's armour and motorized forces were concentrated in Gruppe von Kleist, and sent through the Ardennes. The French did the same with their DLMs, concentrating them in the 7th Army; unfortunately, Paul Gamelin went against French plans and sent the 7th Army in the opposite direction, to aid the Dutch, denuding the French of a strategic reserve. The Luftwaffe had achieved a coup at the opening of the invasion by hitting the Franco-British air forces on the ground. This prevented the French and British from taking advantage of the massive traffic jams in the Ardennes, and paralyzing Army Group A. Without a reserve to block them, the Panzers made it to the coast, and the rest is history; more experienced and battle-hardened officers, with more radios than the French and British, facilitated German success. Even still the French Army fought indomitably along the Weygand Line and the Maginot Line against the German Fall Rot offensive, and the Battle of France was a costly one for the Luftwaffe.

As for the Germans on the Eastern Front, I'd again recommend you to the current thread "Did the Wehrmacht get anything wrong?"; by October, 1941, Operation Barbarossa had failed in it's objectives, and it was unlikely as everyday passed that the war would be a 'short war' that Germany could win.

Blitzkrieg very much depended on having a fully mechanised army supported by the airforce. The armoured tanks with their very heavy turrets and high manoeuvrability supported by Stuka dive bombers weren't easily halted by the likes of Poland and France who relied very much on weapons of the First World War while many were still even on horseback which won't travel as fast and is more susceptible then the German's Sonderkraftfahrzeug 251 AFVs.

The aforementioned Stuka's made quite a frightening howling noise due to 'Jericho Trumpets' they had installed which certainly hit allied morale. The Luftwaffe had also received first hand combat experience when they fought on the side of Franco in the Spanish Civil War unlike those they were fighting. They bombed allied positions and cities meaning when the Heer arrived, they had to often only pick of few remaining defenders. A modern example would be calling in a predator drone or Apache airstrike into a town before entering it.

Training wise, I don't have concrete figures but I'm almost certain many of the soldiers would formerly have been apart of the Hitler Youth since it was mandatory for males between 14 and 18. The German army had access to Soviet territory for training up until the Nazis took over so that gave them an edge over those they were fighting.

I think a lot of it just boils down to the Germans being the aggressors and thus they could decide to attack when they felt they were ready. On the other hand, the allies were desperate for peace and due to most of them slugging behind the Nazis in terms of economic recovery from the depression, rearmament on a massive scale was out of the question until they had no choice. Similarly to how the allies dictated the terms of the Treaty of Versailles, the Nazis more or less dictated when and how the war was going to be fought.