Strictly speaking, it would be correct to date Anderson's career as beginning in September 1944, when AMAZING published his first short story, "A Matter of Relativity." However, dating the beginning of the early phase of his career to the publication of "Tomorrow's Children" (ASTOUNDING, March 1947) is more realistic. Because Anderson only began to write regularly from 1947 onwards.
I argue for dating Anderson's early phase from 1947 when "Tomorrow's Children" (which became the first part of TWILIGHT WORLD) was published. And I date the end of this early phase in Anderson's career to 1958, when THE ENEMY STARS was published. This early phase was when Anderson was still learning how to write, to find his natural voice as a writer, and when he began writing about many of the ideas and themes dearest to his mind. This early period is also when we can detect a few false starts, or perhaps merely a change of mind in how he thought about and wrote his stories. The clearest example of that being the Psychotechnic stories (found in collections such as THE PSYCHOTECHNIC LEAGUE, COLD VICTORY, STARSHIP, and novels like VIRGIN PLANET and THE PEREGRINE).
One of the false starts I believe can be found in Anderson's early phase is "Genius" (ASTOUNDING SCIENCE FICTION, December 1948). I base this on comments by a critic whose name I cannot convincingly recall (it may have been Sandra Miesel) who argued this very early story contradicted the moral values and beliefs of Poul Anderson. I wish I could cite the author by name and quote the exact text. I apologize for this vague and unsatisfactory paragraph and hope I can replace it if I find the text I am incompletely remembering.
Besides hard science fiction Poul Anderson also wrote a smaller but still impressive amount of fantasies, both novels and short stories. The most significant example of that, during his early phase, being THE BROKEN SWORD (Abelard: 1954). It's interesting to note how he became dissatisfied with the original form of that novel and published a revised version 1971. Which means THE BROKEN SWORD can be found in both his early and middle periods. The following bit from Anderson's "Foreword" to the 1971 Del Rey/Ballantine Books edition of THE BROKEN SWORD gives us some understanding of why he became dissatisfied with the first version: "I would not myself write anything so headlong, so prolix, and so unrelievedly savage. My vein is more that of THREE HEARTS AND THREE LIONS."
Going back to the Psychotechnic stories, in an "Author's Note" that Anderson placed at the end of THE PSYCHOTECHNIC LEAGUE (New York, Pinnacle Books, 1981) we are given some comments about his first "future history" and why he eventually became dissatisfied with it. On page 284 Anderson wrote:
A good reason for this abandonment was that the real world had, predictably, not been behaving as described. For example, World War Three remains ahead of us, rather than behind. No doubt I could have fudged my dates a bit. However, I could not explain away important scientific discoveries and technological advances which I had failed to foresee.I date Anderson's middle period as beginning with the publication of WE CLAIM THESE STARS! (Ace, 1959). This middle period is marked by the confidence and strength with which Anderson wrote. Two of his most prominent series of stories which began in his early phase, the stories featuring the Polesotechnic League/Terran Empire and the Time Patrol, reached their full maturity in this middle phase (although Anderson wrote one last Time Patrol story late, in 1995). I would date the end of this middle period to 1989, when THE BOAT OF A MILLION YEARS was published. BOAT shows both definite similarities with Anderson's earlier works and touches on the ideas and themes which would dominate the works he wrote during the last twelve years of his life.
People and institutions had also changed profoundly, as had my view of them. Once I was a flaming liberal, a fact which is probably most obvious in "Un-Man." Nowadays I consider the United Nations a dangerous farce on which we ought to ring down the curtain. (In justice to it and myself, though, please remember that when I wrote this novella the U.N. had quite a different character from that it has since acquired, and looked improvable.)
David G. Hartwell contributed a prefatory essay to the fourth volume of NESFA Press' reprinting of many of Anderson's shorter works in ADMIRALTY: THE COLLECTED SHORT WORKS OF POUL ANDERSON (2011). What he said on page 10 admirably describes qualities which can be found in the stories Anderson wrote during his middle period (in fact, in all three phases). Hartwell wrote: "Instead, again in the hard SF tradition, he most often wrote about strong men and women pitted against the challenge of survival in the face of the natural universe. Some of them die. But Anderson was optimist enough to see beyond the dark times into both a landscape, sometimes a starscape, and a future of wonders--for the survivors. Anderson's future is not for the lazy or the stay-at-homes. He was fairly gloomy about current social trends, big government, repression of the individual, so he catapulted his characters into a future of new frontiers, making them face love and death in vividly imagined and depicted environments far from home. I recall the power and beauty and pathos of his fine black hole story, "Kyrie," the wit of THE MAN WHO COUNTS (THE WAR OF THE WING MEN) the good humor of "A Bicycle Built for Brew," the enormous scope and amazing comprehension of "Memorial." His range was impressive."
(Hartwell's mentioning of "Memorial" puzzles me, I can't find it among Anderson's works. The item closest to it being "In Memoriam," which can most conveniently be found in ALL ONE UNIVERSE, published by Tor Books in 1996.)
And I would date the beginning of Anderson's middle period in his writing of fantasies to the publication of THREE HEARTS AND THREE LIONS in 1961. However, since this edition was only an expanded version of the original form of the novel first published by the MAGAZINE OF FANTASY AND SCIENCE FICTION in September/October 1953, this story belongs more to Anderson's early phase. A truer representative of Anderson's work in fantasies dating to his middle phase is A MIDSUMMER TEMPEST (Doubleday, 1974). An especially interesting thing to note about this book is how it was written almost entirely in blank verse, the form of poetry used by William Shakespeare for his plays. In other words, TEMPEST was written as an act of homage to the Bard.
I am convinced Poul Anderson was a master short story writer. In both fantasy and hard science fiction. By turns poetic and elegiac, and scrupulously faithful to known science or not too impossible extrapolations from what was known. He also excelled in describing his characters and the backgrounds of his stories.
What were some of the ideas and themes which Anderson took up with, in my opinion, magnificent success, in his later years? Immortality, artificial intelligence based on computer technology (AI, for short), the uploading of human personalities into computer networks (and their downloading into human bodies created for them via DNA engineering and cloning), nanotechnology, even raising animal species to human levels of intelligence, etc. Albeit, as of this writing, we are seeing results in the actual world only in cloning and nanotech. I am skeptical some of the themes Anderson speculated about in his later years will ever actually come to pass, such as immortality and AI.
One of the ideas which came most strongly to me as marking Anderson's late phase was how WELL he wrote during the period 1989-2001. It is my opinion that THE BOAT OF A MILLION YEARS marks both the end of his middle phase and the beginning of his late period. These years shows Anderson as not being content to rest on his laurels and rehash old ideas and themes from his earlier years. Instead, his late phase is marked by how boldly he tried out new ideas, some of them very strange to me! I refer, of course, to his four HARVEST OF STARS books, STARFARERS, GENESIS, and the posthumously published FOR LOVE AND GLORY. I ardently recommend readers to try out the HARVEST OF STARS books, despite the difficult ideas found in them (some of which, as noted above, I am skeptical will ever actually come to pass).
One of the themes which marked Anderson's later years was how he preferred to speculate about STL means of mankind reaching the stars. Mostly, of course, because that was, given our current knowledge of science, more likely than having a FTL drive. But he did use FTL for his last novel, FOR LOVE AND GLORY.
Compared to his early and middle phases, Anderson wrote fewer fantasies during his late period, 1989 to 2001. The first being "Faith" (co-authored with Karen Anderson) published in AFTER THE KING (1992). And the first of only two fantasy novels he wrote during his later years was WAR OF THE GODS (Tor: 1997). Truth to say, I consider WAR to be one of Anderson's very few weaker books. The second being OPERATION LUNA (1999), placed in the same "world" as OPERATION CHAOS.
I should also note that during his later years Anderson continue to write short stories, both hard SF and fantasy. Examples being "Death and the Knight," and the posthumously published "Pele" (set in Larry Niven's Man/Kzin wars series) and "The Lady of the Winds" (set in the Thieves World fantasy series).