The literal meaning of "uniform" denotes that each country involved in war must identify itself as belonging to that country so as not to kill or harm its own people.  Yet, in the wearing of their country's uniform, others in another uniform contradict the meaning of the word since they are set apart and identified as different--the enemy. This contradiction of the uniformity of man is what James Kirkup's poem objects so as the poet suggests that all men are uniform themselves in the sense that they are "aware of sun and air and water" and they share humanity:

In every land is common life

That all can recognize and understand.

Let us remember, whenever we are told

To hate our brothers, it is ourselves....

All men are uniform in that they share the essence of being human, living and loving just as others do.  A conscientious objector in World War II, Kirkup makes an appeal to people with his metaphoric use of the word uniform to understand the universal brotherhood of man in his poem, "No Men Are Foreign."

I believe that Kirkup is using the word "uniforms" as a metaphor for all the things that we "put on" to make ourselves different from other people of the world.  These are especially necessary during wars because war is the time when we are most trying to differentiate ourselves from other people.

In this poem, Kirkup is trying to say that people are all essentially the same.  But, he says, we put on these "uniforms" to make ourselves seem different.  We call ourself Christian or Muslim, we support one cricket team or football team or whatever other kind of team.  We see ourselves as American or English or Chinese.  We seem to have an innate tendency to do this.

In war, it is especially important to tell one side from the other.  War is the time when we are trying hardest to be different.  In war we are saying that our kinds of people are so different that we must kill each other.  When we do this, we must be sure that we are making ourselves be as different as possible because we would not want to kill if we thought the other people were just like us.