“Strategically we should despise all our enemies, while tactically we should take them all seriously.”

Table-Talk (1857)
Source: The Complete Works of Henry Wadsworth Longfellow

Actually Henry Wadsworth Longfellow, Driftwood (1857)
Misattributed

Source: Terminal World (2010), Chapter 22 (p. 417)

Fánshì dírén fǎnduì de, wǒmen jiù yào yǒnghù; fánshì dírén yǒnghù de, wǒmen jiù yào fǎnduì.
If the enemy opposes, we must support it; if the enemy supports it, we must oppose it.
Chapter 2 https://www.marxists.org/reference/archive/mao/works/red-book/ch02.htm, originally published in Interview with Three Correspondents from the Central News Agency, the Sao Tang Pao and the Hsin Min Pao (September 16, 1939), Selected Works, Vol. II, p. 272.
Quotations from Chairman Mao Zedong (The Little Red Book)

Often attributed to Stalin, there is not a single source which show that Stalin said this at any given time. There is only one source outside the blogosphere which attributes the quote to Stalin, but does not provide any evidence for the attribution. That source is the book Quotations for Public Speakers : A Historical, Literary, and Political Anthology (2001), p. 121 by the former US senator Robert Torricelli.
Misattributed

1950s, What Desires Are Politically Important? (1950)
Context: We love those who hate our enemies, and if we had no enemies there would be very few people whom we should love.
All this, however, is only true so long as we are concerned solely with attitudes towards other human beings. You might regard the soil as your enemy because it yields reluctantly a niggardly subsistence. You might regard Mother Nature in general as your enemy, and envisage human life as a struggle to get the better of Mother Nature. If men viewed life in this way, cooperation of the whole human race would become easy. And men could easily be brought to view life in this way if schools, newspapers, and politicians devoted themselves to this end. But schools are out to teach patriotism; newspapers are out to stir up excitement; and politicians are out to get re-elected. None of the three, therefore, can do anything towards saving the human race from reciprocal suicide.

Remarks by the President In Photo Opportunity with the National Security Team http://georgewbush-whitehouse.archives.gov/news/releases/2001/09/20010912-4.html, September 12, 2001
2000s, 2001

The True Believer (1951), Part Three: United Action and Self-Sacrifice
Context: It is easier to hate an enemy with much good in him than one who is all bad. We cannot hate those we despise. The Japanese had an advantage over us in that they admired us more than we admired them. They could hate us more fervently than we could hate them. The Americans are poor haters in international affairs because of their innate feeling of superiority over all foreigners. An American's hatred for a fellow American (for Hoover or Roosevelt) is far more virulent than any antipathy he can work up against foreigners. It is of interest that the backward South shows more xenophobia than the rest of the country. Should Americans begin to hate foreigners wholeheartedly, it will be an indication that they have lost confidence in their own way of life. <!-- p. 96

Book II, Chapter 10
From St. Athanasius' Life of St. Antony