The past perfect refers to a time before whenever the simple past time is. That sentence would only occur in a context where that simple past time is already clear -- let's say, August 1939.
With this framework, sentence (1) becomes very straightforward. Take this sentence with no past perfect:
By August 1939, most Europeans knew that war was inevitable.
The phrase in bold can be swapped out for the original phrase without affecting the grammar, and roughly preserving the meaning:
By August 1939, most Europeans had come to accept that war was inevitable.
So the past perfect in (1) means that coming to accept the inevitability of war happened before August 1939, because by that time, they knew.
The inevitability of war itself isn't actually mentioned in the sentence, only the acceptance of the inevitability of war. It's possible to accept something as true even though it is untrue, and it's possible to come to accept something long after it has become true. So while that sentence strongly implies that war was indeed inevitable, it doesn't directly say that.
Your sentence (2) is grammatically correct, but has a different, unusual meaning.
With "would", being the past form of "will" in that context, it sounds like the opinion of Europeans was that war would eventually be inevitable. This is an odd thing to say since it means the inevitability of war itself was inevitable.