“When there is no enemy within, the enemies outside cannot hurt you.”

As quoted in Isaac Asimov (1977) by Joseph D. Olander and Martin Harry Greenberg, p. 165 http://books.google.com/books?id=8HCwAAAAIAAJ&q=%22Babies+are+the+enemies+of+the+human+race%22&dq=%22Babies+are+the+enemies+of+the+human+race%22&hl=en&sa=X&ei=i9ldVKqOM_jLsATbvYCgDA&ved=0CCwQ6AEwAw
General sources
Context: We cannot afford enemies any more … Within a generation or two human society will be in total destructive disarray. Heaven knows how bad it will be. The most optimistic view I can take is this: Things will get so bad within a dozen years that it will become obvious … that we must, whether were like each other or not, work together. We have no choice in the matter. … Technologically, we can stop overpopulation, but we have to persuade people to accept the technology. … Babies are the enemies of the human race … Let's consider it this way: by the time the world doubles its population, the amount of energy we will be using will be increased sevenfold which means probably the amount of pollution that we are producing will also be increased sevenfold. If we are now threatened by pollution at the present rate, how will we be threatened with sevenfold pollution by, say, 2010 A. D., distributed among twice the population? We'll be having to grow twice the food out of soil that is being poisoned at seven times the rate.

De Re Militari (also Epitoma Rei Militaris), Book III, "Dispositions for Action"
Context: It is the nature of war that what is beneficial to you is detrimental to the enemy and what is of service to him always hurts you. It is therefore a maxim never to do, or to omit doing, anything as a consequence of his actions, but to consult invariably your own interest only. And you depart from this interest whenever you imitate such measures as he pursues for his benefit. For the same reason, it would be wrong for him to follow such steps as you take for your advantage. (General Maxims)

Source: Rules for Radicals: A Practical Primer for Realistic Radicals (1971), p. 126–127

Source: Crocodile Tears

There are those who would stop at nothing to injure our economy and our defence. The price of liberty is still eternal vigilance. I know what a fine part the trade unionists of this country have played in our recovery effort. When they are asked to take unofficial action, which may hurt this country, let them just consider carefully whether the motives of those who ask them to strike are really concerned with the interests of the workers.

Broadcast (30 July 1950), quoted in The Times (31 July 1950), p. 4
Prime Minister

1950s, Loving Your Enemies (November 1957)
Context: Now there is a final reason I think that Jesus says, "Love your enemies." It is this: that love has within it a redemptive power. And there is a power there that eventually transforms individuals. That’s why Jesus says, "Love your enemies." Because if you hate your enemies, you have no way to redeem and to transform your enemies. But if you love your enemies, you will discover that at the very root of love is the power of redemption. You just keep loving people and keep loving them, even though they’re mistreating you. Here’s the person who is a neighbor, and this person is doing something wrong to you and all of that. Just keep being friendly to that person. Keep loving them. Don’t do anything to embarrass them. Just keep loving them, and they can’t stand it too long. Oh, they react in many ways in the beginning. They react with bitterness because they’re mad because you love them like that. They react with guilt feelings, and sometimes they’ll hate you a little more at that transition period, but just keep loving them. And by the power of your love they will break down under the load. That’s love, you see. It is redemptive, and this is why Jesus says love. There’s something about love that builds up and is creative. There is something about hate that tears down and is destructive. So love your enemies.

Ein schwacher Feind in der Festung ist fürchterlicher als der stärkste von außen.
Ardinghell and the Blessed Islands (1787).