Filling some gaps about the first war (part 1)
This will be the first of some posts attempting to fill gaps in canon. In some places I rely closely on canon, in others canon is silent and I try to connect whatever dots I find, somehow.
Tom's return to Britain:
We are given the impression that Tom Riddle's job interview with Albus Dumbledore took place not long after Tom's return from 10 years on the continent ("I have come a long way"). Yet we are also told that by then he already commanded a group known as Death Eaters, and this group was originally composed of his old schoolmates. So how were they able to organize so shortly after his return? Well, perhaps the core of the group existed throughout Tom's absence (and may have existed continuously since his school days, through his days working for Borgin and Burkes). Maybe some of them would come to visit him for get-togethers and workshops in the Dark Arts. They could kidnap some Muggles to use as practice subjects, thus raising the unpleasant rumors - rumors about which both Tom and Albus agreed indicated Tom had been pushing the envelope of the Dark Arts.
The original Death Eaters included a Mulciber and a Rosier, people who share surnames with students in Severus' social circle in his younger years. I doubt that a father of a young boy would be inclined towards repeated travel, leaving his family behind. So if Mulciber and Rosier were the fathers of Severus' friends by the same respective surnames then I would guess Tom returned to Britain before the latter were born, sometime in the late 1950s. (Of course, if the younger Mulciber and Rosier were nephews or other relatives of the Mulciber and Rosier of the original group this reasoning does not apply.)
Going with the late-1950s return - why did Tom choose this time to return? He certainly wanted to return, and for more than just hops to hide his Horcruxes in 'significant' locations and perhaps to complete his collection (probably intended with the sword of Gryffindor, but that never worked out). In Britain he could use the Slytherin network to recruit more followers, and more importantly, to ensure that some of his followers are in positions that are useful to him - such as various Ministry departments.
His followers from his school days were dropping out of their meet-ups as they were settling down and starting families. He needed to establish himself close enough if he wanted to keep using them. And this move would also allow him access to the next generation. But he did not want to be connected to his earlier persona - not even by the second generation followers. How fortunate that the Dark Magic changed his appearance beyond recognition! (Or perhaps there was more design in his transformation?) Waiting for his appearance to change sufficiently defined the earliest time he could return. So the earliest he could return was when he was no longer recognizable, but he couldn't wait too long or he'd be abandoned by too many schoolmates. (He had to think of a more reliable way to keep people around.)
So what did he do in those pre-1970 years? Well, he had the opportunity to reunite with old admirers who did not visit him in Europe, or those who dropped out of such activities early on - they were by now established in their careers, and some of them had sons, the oldest of whom were entering Hogwarts. So there must have been much networking - getting involved with those families, exciting the sons with what they were going to achieve by joining him once they were grown. I'm guessing he did not seek to recruit his old schoolmates beyond those who stayed with him all along but focused on getting the sons who did not know anything of his background (perhaps their parents preferred to go along with the Lord Voldemort persona and never mentioned Tom Riddle the orphan because they saw the latter as politically disadvantageous). These sons were starting to leave Hogwarts in the mid-sixties. So I'm thinking Tom's program was to teach them Dark Arts in their free time, while they started careers wherever. During this time Tom remained under the radar so I'm guessing his victims were mostly Muggles.
Of course out of anyone's sight Tom was working on hiding his Horcruxes. He placed the diadem in the ROR the night of the interview. I'm guessing he next was getting hold of the Riddle House (Imperiurising the owners to sell it to him for pennies?), place the cursed ring in a golden box under the floor of the Gaunt hovel (with the addition of a Muggle-repelling charm on the shack). After that he started working on a more sophisticated death trap in his favorite cave, for his favorite Horcrux.
What was Albus thinking all this time? For this I must thank Terri for pointing out that at this point Albus was not concerned with Tom possibly having a Horcrux but with his connection to one of the Hallows, the Resurrection Stone. In the 1940s Tom showed up at school with a ring that had the mark of the Hallows in its stone. It may have seemed like a piece of Hallows Quester's jewelry, but perhaps at some point Albus suspected it was the actual Hallow. Because after 5 years of procrastination he finally went to meet Gellert. In part - because knowing Tom had the Stone meant that Gellert was not yet Master of Death and Albus stood a chance against him. And in part - out of fear that Tom would find out and go to Gellert himself - and Albus would end up fighting whichever of them ended up with (at least) 2 Hallows.
So how do Tom's actions look via a Hallows-tinted lens? He leaves school and goes to work in Knockturn Alley, not far from Ollivander's store. After a few years he goes to Europe and when he returns first thing he wants an interview with Albus. No doubt he learned from Ollivander about the rumor that Gregorovich had the Elder Wand and set for the continent at his convenience (once he also had Madam Smith's trinkets). Gregorovich was of little help because the wand had been stolen from him by some young man only few could recognize as Dark Lord Grindelwald. So Tom must have spent much time researching until he figured out what happened - and came right to the office of the Master of the Elder Wand. It all made sense! Tom must have come for Albus' wand! The entire conversation was a dance for domination between the two of them - and then Tom looked like he was reaching for his wand as he left. Was he trying for Expeliarmus? Accio Dumbledore's wand?
Of course Tom had no idea about the Hallows. But perhaps he was thinking he could Accio Gryffindor's sword, if it was anywhere around.
And then Tom spends years not showing any sign of being more than a 'coach' of a Dark Arts club. And definitely no sign of being in possession of a most powerful Dark artifact. So Albus waited, in puzzlement.
Tom's return to Britain:
We are given the impression that Tom Riddle's job interview with Albus Dumbledore took place not long after Tom's return from 10 years on the continent ("I have come a long way"). Yet we are also told that by then he already commanded a group known as Death Eaters, and this group was originally composed of his old schoolmates. So how were they able to organize so shortly after his return? Well, perhaps the core of the group existed throughout Tom's absence (and may have existed continuously since his school days, through his days working for Borgin and Burkes). Maybe some of them would come to visit him for get-togethers and workshops in the Dark Arts. They could kidnap some Muggles to use as practice subjects, thus raising the unpleasant rumors - rumors about which both Tom and Albus agreed indicated Tom had been pushing the envelope of the Dark Arts.
The original Death Eaters included a Mulciber and a Rosier, people who share surnames with students in Severus' social circle in his younger years. I doubt that a father of a young boy would be inclined towards repeated travel, leaving his family behind. So if Mulciber and Rosier were the fathers of Severus' friends by the same respective surnames then I would guess Tom returned to Britain before the latter were born, sometime in the late 1950s. (Of course, if the younger Mulciber and Rosier were nephews or other relatives of the Mulciber and Rosier of the original group this reasoning does not apply.)
Going with the late-1950s return - why did Tom choose this time to return? He certainly wanted to return, and for more than just hops to hide his Horcruxes in 'significant' locations and perhaps to complete his collection (probably intended with the sword of Gryffindor, but that never worked out). In Britain he could use the Slytherin network to recruit more followers, and more importantly, to ensure that some of his followers are in positions that are useful to him - such as various Ministry departments.
His followers from his school days were dropping out of their meet-ups as they were settling down and starting families. He needed to establish himself close enough if he wanted to keep using them. And this move would also allow him access to the next generation. But he did not want to be connected to his earlier persona - not even by the second generation followers. How fortunate that the Dark Magic changed his appearance beyond recognition! (Or perhaps there was more design in his transformation?) Waiting for his appearance to change sufficiently defined the earliest time he could return. So the earliest he could return was when he was no longer recognizable, but he couldn't wait too long or he'd be abandoned by too many schoolmates. (He had to think of a more reliable way to keep people around.)
So what did he do in those pre-1970 years? Well, he had the opportunity to reunite with old admirers who did not visit him in Europe, or those who dropped out of such activities early on - they were by now established in their careers, and some of them had sons, the oldest of whom were entering Hogwarts. So there must have been much networking - getting involved with those families, exciting the sons with what they were going to achieve by joining him once they were grown. I'm guessing he did not seek to recruit his old schoolmates beyond those who stayed with him all along but focused on getting the sons who did not know anything of his background (perhaps their parents preferred to go along with the Lord Voldemort persona and never mentioned Tom Riddle the orphan because they saw the latter as politically disadvantageous). These sons were starting to leave Hogwarts in the mid-sixties. So I'm thinking Tom's program was to teach them Dark Arts in their free time, while they started careers wherever. During this time Tom remained under the radar so I'm guessing his victims were mostly Muggles.
Of course out of anyone's sight Tom was working on hiding his Horcruxes. He placed the diadem in the ROR the night of the interview. I'm guessing he next was getting hold of the Riddle House (Imperiurising the owners to sell it to him for pennies?), place the cursed ring in a golden box under the floor of the Gaunt hovel (with the addition of a Muggle-repelling charm on the shack). After that he started working on a more sophisticated death trap in his favorite cave, for his favorite Horcrux.
What was Albus thinking all this time? For this I must thank Terri for pointing out that at this point Albus was not concerned with Tom possibly having a Horcrux but with his connection to one of the Hallows, the Resurrection Stone. In the 1940s Tom showed up at school with a ring that had the mark of the Hallows in its stone. It may have seemed like a piece of Hallows Quester's jewelry, but perhaps at some point Albus suspected it was the actual Hallow. Because after 5 years of procrastination he finally went to meet Gellert. In part - because knowing Tom had the Stone meant that Gellert was not yet Master of Death and Albus stood a chance against him. And in part - out of fear that Tom would find out and go to Gellert himself - and Albus would end up fighting whichever of them ended up with (at least) 2 Hallows.
So how do Tom's actions look via a Hallows-tinted lens? He leaves school and goes to work in Knockturn Alley, not far from Ollivander's store. After a few years he goes to Europe and when he returns first thing he wants an interview with Albus. No doubt he learned from Ollivander about the rumor that Gregorovich had the Elder Wand and set for the continent at his convenience (once he also had Madam Smith's trinkets). Gregorovich was of little help because the wand had been stolen from him by some young man only few could recognize as Dark Lord Grindelwald. So Tom must have spent much time researching until he figured out what happened - and came right to the office of the Master of the Elder Wand. It all made sense! Tom must have come for Albus' wand! The entire conversation was a dance for domination between the two of them - and then Tom looked like he was reaching for his wand as he left. Was he trying for Expeliarmus? Accio Dumbledore's wand?
Of course Tom had no idea about the Hallows. But perhaps he was thinking he could Accio Gryffindor's sword, if it was anywhere around.
And then Tom spends years not showing any sign of being more than a 'coach' of a Dark Arts club. And definitely no sign of being in possession of a most powerful Dark artifact. So Albus waited, in puzzlement.