In 1984, according to chapter 3 of Goldstein's book, why is war peace?

You can find the answer to this question in book 2,
chapter 9. Essentially, continuous war keeps the citizenry equally dependent on the
government for their own rations of goods. Likewise, war means the people are united in
purpose against an enemy other than the government and their is no room for large-scale
revolt. A couple of further reasons that war is peace are listed below as direct quotes
from the book:

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War is a way of shattering to pieces... materials
which might otherwise be used to make the masses too comfortable, and hence, in the long
run, too intelligent.

If
everyone is dumb, they will live without planned purpose to revolt. This will mean
peace.

Another way that war is peace must be deduced by the
reader. By reading chapter 3 of Goldstein's book, readers learn that the three
superpowers go to significant lengths to put on a pretend war to keep the people
believing. This great collaborated effort must take much time and tact to orchestrate.
Working together so much on a common purpose quite frankly prevents the need for real
war.

The war,
therefore, if we judge by the standards of previous wars, is merely an imposture...
though it is unreal it is not
meaningless.

Goldstein admits
that war is fake.

One good quote that directly states why
war is peace is actually in chapter 1 of Goldstein's
book:

...
continuous warfare ... is also useful in keying up public morale to the necessary
pitch.

This points out that
the public grows in their allegiance and degree of happiness or peace as long as there
is war.