Perhaps if the US hadn't waited so long to enter WW2 in the first place, the war wouldn't have escalated as much as it did.

Should the US have launched a pre-emptive strike on Japan, or was it right to wait for Japan to attack it first?

For that matter, was it right for the US to sit on the sidelines and watch Germany attack the rest of Europe, and dither about getting involved?

Or even for the UK to wait for Poland to be invaded?

Is there some sense to pre-emptive strikes, or are they always wrong? Do they save lives, or cause unnecessary killing?