In episode 6 of the TV adaption of The Man In The High Castle, Obergruppenführer Smith and Rudolph Wegener talk about the time they fought in war, implying even Nazi war crimes. However, John Smith is an American native who doesn't really speak German.
The answer is probably not told, but how could he manage to join the Nazi army?
I'm not sure which particular conversation from episode six you're talking about. I have found only one in which they discuss the war (00:33:07):
Wegener says:
Nobody talks about the camps. Nobody talks about how many we exterminated. We got commendations for it. But nobody talks about it.
By "we" here, Wegener could simply be referring to himself and Nazis in general.
However, Smith responds:
What's there to say, Rudolph? It was necessary work. We did it.
Which implies that Smith also took part in such atrocities. This however, leaves three options.
Smith joined the Nazis early, perhaps traveling to Germany (As in the answer of @NKCampbell or something similar.) He thus took part in atrocities in Europe.
After the US surrendered, Smith joined the Nazis as white americans were encouraged to do under the new regime. The camps and exterminations Smith took part in were thus all after the US had surrendered. Smith may refer to this as part of the "war" as he was dealing with a (probably initially large) American civilian resistance, etc, which was probably almost like a minor civil war initially. Another reason he refers to it as a "war" could be the fact that he is trying to justify his actions. (Wegener, who brought it up, had not used the word "war" while talking about the atrocities.)
Throughout the series, Smith is often talking about the individual being unimportant and needing to devote itself to the greater cause of Nazism. Thus, when he says "We did it", he may simply be talking about the Nazis as a whole, seen as a giant entity which he considers himself a part of. Though, of course, we also know that Smith has done plenty of terrible things himself during his career in the SS.
Regarding option #1, one thing to note is that Smith's German is limited. In episode 8, Heydrich asks Smith whether his children speak German. Smith replies
Better than me, certainly.
Heydrich then says
Ah. I seem to recall.
This might be inconsistent with Smith staying several years in Germany surrounded by German speakers? Though not necessarily. It's possible he lived there without learning German too well. I just figured it was worth noting.
In Season Two, it is definitively revealed that he was with the Americans in WWII.
In S2E05, he meets with Inspector Kido who questions him about a medal displayed in his office:
Kido: The medal behind your desk. Might I examine it more closely?
Smith: Be my guest.
(Kido gets a closer look at the medal, which is dated 1942)
Kido: The Solomon Islands. Why display a medal from your U.S. military service?
Smith: I keep it as a reminder. The consequences of the failure of command.
Kido: There were many casualties of that campaign. On both sides.
In S2E10, it's revealed that he witnessed (from afar) the nuclear attack on Washington D.C. on December 11, 1945. He can be seen wearing his uniform.
Just prior to this event, he spoke to his (then pregnant) wife of going to the Pentagon:
Helen: You were supposed to be there hours ago.
John: Ah, that's fine, I'll just go straight to the Pentagon in the morning.
So while it's possible he had Nazi sympathies before the war, he did serve on the U.S. side until the surrender. Because of the nuclear attack, that he escaped, he may well have been one of the highest ranking military officers in the region and a swift capitulation might have seemed to be the best route to ensuring the continued safety of his family.
Speculation:
I'm reminded of the scene from the HBO Miniseries "Band of Brothers"
I'm from Eugene, Oregon too.
Why are you in a Kraut uniform?
Volksdeutsche.
Come again?
My family answered the call. All true Aryans should return to the Fatherland.
From the Wikipedia article re: Volksdeutsche
"During the early years of the Second World War (i.e., before the US entered the war), a small number of Americans of German origin returned to Germany; generally they were immigrants or children of immigrants, rather than descendants of migrations more distant in time. Some of these enlisted and fought in the German army."
Thus, it is historically accurate to presume that perhaps Smith or his family returned to Germany at the outbreak of WWII, he rose in the ranks, and was transfered back to the US to assist in the occupation of his native home.
I think it's obvious that he's referring to running the internal US extermination camps for jews and other "state enemies" while the Germans were still fighting the Russians. When did they say Stalin was executed? Early 1950's? I forget. Anyway German/Russian war was ongoing when the US was conquered so at that point they became allies and Nazis started rounding up people for extermination.
In one of the episodes, It shows Captain Smith, Signal Corps, US Army, in a Stateside assignment, after the US surrender. His Commander brings in a bunch of NAZI armbands, where he and some of the other officers and soldiers are having drinks (lamenting the surrender???), and hands them out as the only way to the future. Also, there is the episodes where he keeps the campaign medal from service in the Pacific. Overall Captain is not a high rank. He could have been a signal officer for a regiment (Staff Position), or commanded a signal company. This would be a typical rank for an officer that joined in late 41 or early 42. Being a Signal Corps Officer, there would not have been the rapid promotions as say the infantry (Such as Major Winters, who went from 2LT to MAJ during WW2). His decorations are all post WW2 under the Greater Reich. And, it can be expected that there would great resistance to such occupation. Further, if DC was hit by a Nuc, there would have been no government. However, it's one thing to Nuc a capital, either by bomber or missile, and another thing to occupy said country. So it is quite conceivable that Obergruppenfurher Smith received numerous decorations from the Greater Reich during the subjugation of the American Continent East of the Neutral Zone. There would have been plenty of opportunities for rapid advancement and promotion.
There is a scene where they are in Smith's office and in the background of Smith's office is an American Intelligence medal from the Pacific.
Please correct me if I'm wrong, but I seem to remember Smith saying he was an Lt. in the American Army at the beginning of WW-2. England fell to the Germans early in the War (this version,) and when the Americans realized they were going to lose to the Japanese Caucasian Americans joined the Nazi's to fight the Soviets.
