Jay Honeck wrote:

Thus, Pearl Harbor truly marked the first day in a long march toward
Austria's freedom...

There was no question that the US participation was critical to the end
of the war, and December 7 was the start of major American
participation. But I look at it more as a turning point in the overall
struggle against the Nazis. The force of US industry combined with
fresh troops was certainly necessary to swing the course of the war,
which had been very one-sided in favor of Germany up to that point.

However, to say that it was the start or the first day diminishes the
role of those people who had been struggling, and dying in large numbers
in the battle against the Nazis for the previous two years. They had
been engaged in fighting on their home soil, and it was a struggle for
survival. The British had already rebuffed the Luftwaffe in the Battle
of Britain, and maintained superiority over the seas with the sinking of
the Bismarck.

The eastern front was opened up only six months prior to the attack on
Pearl Harbor with the invasion of Russia, and you could equally say that
the war would not have ended as it had without Russian participation.
Their subsequent losses were staggering in comparison to the other
allies, and they contributed huge amounts of armor and soldiers. Some
of the largest battles of the war took place on the eastern front,
involving thousands of tanks and millions of soldiers in single battle
lines. Had these resources not been tied up on the eastern front, it
would have been nowhere near as easy to march against Berlin from the
west.

While the Russians were immediately forced into fight when they were
invaded, the US took some time to mobilize, and it wasn't until almost
another year had elapsed before Allied forces moved against the
Afrikacorps in Morocco and Algeria with US participation. In the
meantime, the Russians endured the siege of Stalingrad, and the
accompanying losses.

There are many events that could be classified as the start of the
resistance against Hitler, and my contention is that the first step was
in the declaration of war against Germany by England and France, since
that signaled the end of the policy of appeasement.