After Adolf Hitler's rise to power, the Nazi Germany started imperialist policy, seizing first Austria and Czechoslovakia in 1938 and 1939, without international opposition. However, the invasion of Poland in 1939 triggered the WWII.
Despite their eventual loss of the war, the armed forces of Germany were armed with many highly advanced tools. The more famous tools in their possessions were undoubtedly their tanks and jet aircraft, both of which the Allies were never really able to fully counter during the conflict.
Germany is able to field more heavily armed and armoured fighting vehicles than their enemies; the Tiger, Panther and Tiger II have no real battlefield counterparts and are extremely dangerous, a fact Allied forces learned quickly during and after the Normandy landings and the liberation of France. For the Tiger and Tiger II inparticular, the only consistent way to deal with their presence during a battle was air support in the form of rocket-firing Mustangs and Typhoons that could attack the heavy tanks from the relative safety of the air. On the ground, British and American tanks were woefully undergunned and underprotected, and suffered huge losses in direct confrontations with heavy German tanks.
Germany's aircraft can also be quite spectacular. Particularly, the Fw 190A-8 superiority fighter is the most heavily armed combat aircraft available, armed with four 20mm cannons and two 15mm cannons, its damage projection far outweighs its main rivals. The Me 262, the first jet aircraft to see widespread service, is even more heavily armed, with four 30mm cannons and racks of air-to-air unguided rockets and speeds that no Allied aircraft can match.
For all of its strength however, German forces have several underlying weaknesses that are easily exploited by its enemies. By 1944, Germany was ruined economically -- most of its industry by this time was utterly destroyed by round-the-clock bombing from Britain and France. Perhaps worst of all, it suffered catastrophic fuel shortages that grounded many of its planes and impeded deployment of many of its tanks. Almost as bad were its manpower shortages; after 5 years of constant warfare much of Germany's available manpower, specifically men between the ages of 16-45, were heavily depleted -- a popular example of the hardships this placed on the German war machine is the fact that production of aircraft and tanks was its highest during 1944, but Germany simply did not have the fuel or the manpower to use them.
This affected the fighting capabilities of the Wehrmacht's frontline units as well. By 1944 much of Wehrmacht combat units were composed of foreign conscripts, POWs, the very young and the very old, a stark contrast to years earlier when the Wehrmacht embodied the idea of the most professional and well-trained army.