In "No Men are Foreign," James Kirkup urges us to put aside what he sees as our superficial differences and recognize that we are all ultimately part of one human race. We all share the same earth; we all belong to it equally. It doesn't matter whereabouts on this planet we may live, what race we are, which flag we salute; we all thrive in peace and are damaged by war.
Yet all too often we're encouraged—usually by the powers-that-be—to hate our fellow man, and for the most trivial of reasons. The end result of this hatred is invariably more conflict, where we are starved by "war's long winter," as Kirkup puts it. But because we are constantly enjoined to hate our brothers, it's all the more essential that we recognize the fundamental unity of humankind. In doing so, we will come to realize that, in hating others, we are also hating ourselves. And as this earth belongs to us all, when we go to war with our fellow man, we are really defiling our own home.
Kirkup's aim in writing "No Men are Foreign" may be to get us to rediscover our common humanity. But he achieves this aim by appealing to our selfishness; he gets us to realize that, in separating ourselves from one another, we're acting against our own best interests.
I think that the title of the work might help to express the theme of universal brotherhood. The idea of all men and people living in a world devoid of boundaries and geographic constraints helps to convey the idea of universality in identity. Cultural and natural barriers to collective solidarity are diminished in the mere theme. The "air" that is everywhere is another element that conveys universal brotherhood, invoking President Kennedy's appeal in that "We all breathe the same air." Kirkup opens the poem with discrediting the military and nationalism that divides human beings from acknowledging one another. The notion of how basic physicality of appearance helps to bring people together is reflected in the poem, conveying the theme of universal brotherhood. The poem is a call to action in transforming the world into what it should be in terms of a realm where all human beings are able to identify with one another and not against one another. This is a declaration of a theme of universal brotherhood evident in the poem from the opening to the closing line and sentiment.