I see it repeated often as a matter of fact that, after a certain point in European (or World, the same is said of Imperial Japan) that expansion through conquest ceased being as enticing a proposition as it was in older times.
The reasons cited include a hostile, nationalist population and sanctions inflicted by the international community but the one I see a lot is the difficulty of taking the production of the conquered territories. For example: Nazi Germany could plunder its conquered territories but they did more damage to the industrial output of those countries by doing so.
How true is this, and was there a definitive turning point where large conquests ceased being viable?
1948 large conquests ceased to be a viable option with the start of the Cold War. The only two nations capable of large conquest, USA and USSR, used proxy states for conquest because they feared direct confrontation might lead to nuclear war.
Any nation that has attempted direct conquest has found itself confronted by overwhelming military intervention. From 1950 when North Korean attempted to conquer South Korea and was quickly overwhelmed by the US led UN force. To 1990 when Saddam Hussein found himself on the wrong end of another US led military machine.
The superpowers themselves weren't able to conquer because of proxy fighters supplied by the opposing superpower. Think Korean War, Afghanistan, Vietnam. The breakup of the colonial empires of GB and France led to several small wars as the new nations tried to define themselves, which for a nation includes defining your borders.
Since the end of the Cold War conquest has not been an option due to the hegemony of the United States and NATO allies.
In terms of industrial output Germany and Japan both required very limited industry from the conquered territories. Japan required rubber and oil from Indo-China and Steel from China. Germany required Oil from Russia and steel from Ukraine. It proved relatively simple for these nations to set up industry around only the raw resources they needed while stripping the rest of the conquered lands of everything valuable.