Kipling's poem "The White Man's Burden" can be considered either fictional or non-fictional depending on the particular message being considered and the perspective from which it is analyzed.
The most obvious answer is that the poem is fictional. "The White Man's Burden" presents imperialism in a highly romanticized form, almost monastic, where the sole motive is altruistic and even self-sacrificing benevolence:
Take up the White Man's burden --
No tawdry rule of kings,
But toil of serf and sweeper --
The tale of common things.
The ports ye shall not enter,
The roads ye shall not tread,
Go make them with your living,
And mark them with your dead!
The money-making and power politics that always provides the main foundation for imperialism is nowhere mentioned. Instead, there is the expensive and tedious "burden" assumed over "thankless years" of trying to "civilize" a pack of ungrateful cads:
Take up the White man's burden --
And reap his old reward:
The blame of those ye better,
The hate of those ye guard --
In reality, this construction of the motivations of imperialism, and its presumed good effects on those ruled, is a self-serving fiction. Empires are not, and have never been, charity concerns.
However, despite its highly romanticized take on empire-building, Kipling's poem also conveys some important truths. The first is that no matter how determined and (self-) righteous, "civilizers" can expect little thanks for their efforts. Kipling probably would have said this is because the subjects of the empire are short-sighted, selfish, and stupid; we would suspect it had far more to do with national pride, cultural difference, and resentment at being ruled by foreigners. All the same, the warning to those who might think imperial "liberators" are always greeted with flowers and cheers is plain to see. Kipling even goes so far as to remind the imperial power that if it takes this path of "principle," it cannot afford to falter in the slightest -- it must remain true to its own rules:
By all ye cry or whisper,
By all ye leave or do,
The silent, sullen peoples
Shall weigh your Gods and you.
Moreover, in stressing the costs of empire -- the "ports" and the "roads" -- Kipling underlines that even if seen romantically, an empire is a tremendous burden to the imperial power.
Thus, in its main thrust, the idealization of Western imperialism, its presentation as nothing more than a sort of altruistic missionary endeavor, "The White Man's Burden" may be classified as fictional. However, in its reminder of the burdens of empire and the uncertainty of some of its rewards, particularly gratitude, it brings forward important factual points. Both messages were highly relevant in 1899, when the poem was written, since the United States (to which the poem was addressed) had just acquired the Philippines.