I suppose this question is more or less inspired by Richard Evans' Third Reich trilogy. A lot of official German propaganda as well as private citizens' thinking at the start of the Second World War played on the idea of restoring 1914 borders and vaguely "restoring" Germany to its rightful place as a power.
It makes me curious: how much of this was based off of feelings of grievance, versus an actual decline in relative strength. I'll stress relative instead of absolute, because pretty much every European power was better off materially and technologically in 1939 than in 1914.
Or does it even make sense to try to make this comparison?
If you look at the armies involved, both sides are relatively close in numbers. If anything, Germany (plus the annexed Austria) is weaker in terms of the number of divisions in 1939 than in it was in 1914.
Note: these are rough numbers - other sources may give different numbers depending on how the count reservists, training units, and colonial units. Also, by May 1940, the armies of both sides have grown considerably.
The belligerents in September 1939:
Axis
Germany: 100 infantry divisions, and 6 armoured divisions
Allies
France: 90 infantry divisions
Great Britain: 25 infantry divisions, 1 armored division = appx. 900,000 men
British India: 4 infantry divisions, 5 cavalry brigades = 200,000 men
Poland: 30 infantry divisions, 30 reserve infantry divisions (too slow to be mobilized in time), 12 cavalry brigades, one armored brigade
The belligerents in August 1914
Central Powers
Germany: 98 divisions, 27 Landwehr brigades = 1,900,000 men
Austria-Hungary: 48 divisions = 450,000 men
Entente Powers
Russia: 102 divisions = 1,400,000 men
France: 72 divisions = 1,290,000 men
Great Britain: 6 divisions, 14 Territorial Army divisions (not-deployable overseas) = 120,000 men
However, all the armies involved are now considerably better-armed and equipped. Consider a rough sketch of how prevalent machine guns became in every European army between 1914 and 1939. In 1914, the British Army had one heavy Maxim machine gun for appx. every 500 riflemen. By 1918, it had about one ligher Lewis Gun for every rifleman. By 1939, it had one even lighter Bren Gun for every 8 (later every 10) riflemen.
The same of course applied to other equipment: trucks, radios, mortars, anti-tank guns, tanks, and much more. A division in 1939 hit much harder than one in 1940. Now, I wouldn't got so far as to say German troops were necessarily better-equipped than their opponents. In some areas, like light machine guns and assault guns, they arguably were. However, in other areas, they unquestionably lagged. In 1940, the BEF was probably the best-motorized force in the world, while the Germans made extensive use of horses for logistics and footsore infantry for combat. As Churchill memorably said on June 4th, 1940:
Behind [the German] armored and mechanized onslaught came a number of German divisions in lorries, and behind them again there plodded comparatively slowly the dull brute mass of the ordinary German Army."
Granted, focusing just on armies doesn't take into account the interwar development of air forces. The Luftwaffe had over 2,000 combat aircraft in 1939, a far cry from the handful of spotter planes they brought into 1914. Germany was vastly more powerful in the air in 1939 and arguably had the best air force in the world at that point.
Germany, however, was much weaker at sea in 1939 than it was in 1939. In 1914, Germany had 14 dreadnaughts and 4 battlecruisers. But Germany only had 2 battleships and 3 pocket battleships in 1939. It had better submarines, but less than 40 ocean-going Type VII U-boats, when Karl Doenitz had wanted the Kriegsmarine to have 300.
Bottom Line: In terms of its ability to wage war in Continental Europe, Germany was stronger in 1939 than it was in 1914. It's performance in 1940 compared to its performance in 1914 is certainly the strongest evidence of that.