How could Heracles ensure the victory of the Olympians against the Titans if he was born mortal?
In ancient greek mythology, Heracles is the hero with the most adventures ever. There is, however, a temporal paradox between the different myths about him that bothers me a bit.
Heracles was born as a mortal son of Zeus. He has done a lot of heroic deeds while traveling around Earth, as well as a few shameful crimes like killing his own wife and children, Kheiron the centaur, and Iphitos.
When Heracles was poisoned by the blood of Nessos, in his unbearable pain, he chose to commit suicide by getting burned alive on a funeral pyre. But before he could die, Zeus has rescued him by raising his spirit to Olympos, making him an immortal god.
Zeus has insisted that Heracles shall become one of the gods who participate in the regular dinner feasts of the gods on Olympos with Zeus himself. As, however, there were previously twelve guests to those feasts, and thirteen was an unlucky number, Zeus has forced one of the other gods to give up their place in favor of Heracles.
Drinking ambrosia in the company of Zeus couldn't entirely satisfy Heracles, so he continued his heroic deeds as an immortal god. Already experienced in defeating immortal beasts, he again helps the Olympians by fighting against the monstrous children the primordial gods have spawned. As a physically strong god, he was a decisive factor in ending the rule of the Titans and ensuring that the Olympian gods would rule the world.
These stories seem contradictory. If Zeus could raise his son to immortal status, and even re-arrange the the seating at the high table or Olympus, then surely he was already the king of Gods, comfortably ruling Earth together with the other Olympians. The fight against the old gods must have happened before that.
What is the correct timeline here? Did Heracles travel back in time after he became immortal?
I could be wrong, but I don't think there's an ancient source mentioning Heracles took part in the Titanomachy. Are you sure you aren't thinking of the Gigantomachy#The_Gigantomachy)?
@yannis That's possible. It wasn't clear to me that these were two separate wars with peace between.
I apologize for the length of this answer, per the background--the two wars are indeed distinct, as Yannis points out. Your point about temporal contradiction is valid, (and in my experience, time can be non-linear in the Greek Myths,) but there is no easy explanation.
"Then Zeus no longer held back his might; but straight his heart was filled with fury and he showed forth all his strength. From Heaven and from Olympus he came immediately, hurling his lightning: the bolts flew thick and fast from his strong hand together with thunder and lightning, whirling an awesome flame. The life-giving earth crashed around in burning, and the vast wood crackled loud with fire all about. All the land seethed, and Ocean's streams and the unfruitful sea. The hot vapor lapped round the earthborn Titans: flame unspeakable rose to the bright upper air"
The Gigantomachy is a subsequent war, by some accounts incited by the casting down of the Titans and Gaia's dissatisfaction with that outcome:
"Earth, vexed on account of the Titans, brought forth the giants".
Source: Apollodorus, Library 1.6.1
The first account of Heracles aiding the gods in this battle against the giants seems to be Pindar, a few centuries after Hesiod:
"[Zeus] stood, possessed by overwhelming astonishment and delight. For he saw the supernatural courage and power of his son"
Source: Pindar, Nemean Odes, 1, line 55
Zeus consults the seer, Teiresias, who prophesies:
"...that when the gods meet the giants in battle on the plain of Phlegra, the shining hair of the giants will be stained with dirt beneath the rushing arrows of [Heracles].
Source: ibid. line 67-68
This has been interpreted to mean that the gods would need the aid of a "mortal" to overthrow the giants (similar, perhaps, to how the Witch King in Tolkien can be killed by "no man";)
Apollodorus very specifically comments on this:
Now the gods had an oracle that none of the giants could perish at the hand of gods, but that with the help of a mortal they would be made an end of.
Source: Apollodorus, Library 1.6.1
Apollodorus provides an example in the death of the giant Alcyoneus:
Hercules first shot Alcyoneus with an arrow, but when the giant fell on the ground he somewhat revived. However, at Athena's advice Hercules dragged him outside Pallene, and so the giant died.
Note that the term "simple" is used in this old-fashioned translation for the actual Greek word "pharmakon", here simply referring to a magic plant, although the term can also mean a sacrificial victim.
It is possible that Heracles' legendary strength may exceed that of any god, in that he is the one who has to drag the giant. The precedent for requirement of brute force may be said to derive from Hesiod's account of the Titanomachy:
"But until then, they kept at one another and fought continually in cruel war. And amongst the foremost Cottus and Briareos and Gyes insatiate for war raised fierce fighting: three hundred rocks, one upon another, they launched from their strong hands and overshadowed the Titans with their missiles, and hurled them beneath the wide-pathed earth, and bound them in bitter chains when they had conquered them by their strength for all their great spirit."
Source: Theogony, 711-719
And thus Heracles involvement is a reflection of the involvement of the Hecatoncheires in the previous war. The idea would be that the Titans and Giants are chthonic entities and cannot be overcome by heavenly forces alone (i.e. the thunderbolts can knock them down, but not kill them. Note that the Titans were not killed but cast down into Tartarus.)
Regarding the temporal anomaly:
If we accept that the time period figures such as Alcmene and Amphitryon came after the Gigantomancy, then Heracles aided the gods before he was born. Dionysus was also said to be involved in the battle per Euripides, and like Heracles, would seem to have been conceived in a later, more settled age.
But Apollodorus doesn't explicitly state when the Gigantomachy occurred, and as the battle is absent in Hesiod, who is more linear, it's difficult to nail down chronologically.
It seems sensible that the Gigantomachy occurred before the ages of man in that there don't seem to be stories of humans being affected by the battle.
I hope this attempt to answer your very salient question is not too unsatisfactory. My personal take, based on years of study, is that time in Greek Mythology is not strictly linear, but occurs in a sort of "eternal past" that is "eternally present".
Here are some scholarly papers on the subject:
I don't understand why the note about the term "simple" because that word never occurs elsewhere in the answer.