Refer to the poem "No Men are Foreign,"and say what shall happen if we take up arms against each other.

To find the answer to this question, you need to refer to
lines 16-19 of the poem.  Basically, Kirkup is saying that we will go against nature if
we take up arms against one another.

What the poet says is
that we will "defile" the earth and "outrage" the air that is around us if we fight. 
But what does he mean by this?  To me, what he is saying is that war -- the killing of
other human beings -- is an act that is against nature.  It is against what we as people
are meant to do.  So when we do it, we are contaminating the earth and the sky
(metaphorically) with the evil of our deeds.

Instead of
this, we are all supposed to live together in harmony because that is what nature
intends for us (since we are all essentially alike).