The linked article on Ted Hall has him explaining why he did it. All such justifications must be taken with a grain of salt, of course, but he claims to have feared that if a post-war depression ensued, the USA could turn totalitarian and fascist, in which case it would be better if the USA did not have a monopoly in nuclear weapons.

He also recounts the extent that his views were shaped by his slightly older roommates at Harvard, whom he was obviously in awe of.

Rather more self-servingly, he claims that he probably would not have spied if he had known how despotic the Soviet Union was.

Money was apparently not involved.

https://www.nytimes.com/1997/09/14/magazine/the-boy-who-gave...

The British during WWII had probably the most successful counter-espionage program in history, because they understood the personal motivations of the people who were interested in spying on them and used those motivations to their advantage. There are many books on the subject, my favorite of which is [0].

[0] https://www.amazon.com/Double-Cross-Story-D-Day-Spies/dp/030...

On the other hand, the British intelligence community was riddled with Soviet agents because of their trust in "the right sort of people" like Kim Philby.

While true, that was a post-War phenomenon, very different from the state of British counter-intelligence during the war.

Read or listen to ‘The Company’ for a fantastic look back at spying during the Cold War. It introduced me to Philby

And Anthony Blunt.

I guess this means that counter espionage is invariably shit? Because the Brits graciously donated Klaus Fuchs[1] to the project, and that was after having him interned at first.

To be fair tough, Fuchs made important enough contributions that the Manhattan project might well have been too late to bomb japan before the war ended.

[1]: https://en.wikipedia.org/wiki/Klaus_Fuchs

I think mister Philby disagrees with you.

Money, Ideology, Compromise+Extortion, Ego.

https://en.m.wikipedia.org/wiki/Motives_for_spying

In the case of the early Los Alamos scientists: Ideology

Ideological sympathies, the Soviet Union was an ally at the time, concern about balance of power given a new superweapon. As rationalizations go, the atomic spies had more to work with than most.

Money rules the world. Robert Hanssen and Aldrich Ames did this purely for the money.

https://en.wikipedia.org/wiki/Robert_Hanssen

https://en.wikipedia.org/wiki/Aldrich_Ames

> As fascinating as I find "how" the spies did it, I'm more interested in "why" they did it? Was it dissatisfaction with something in their lives? For the money (boring)? Because they didn't want to see the US so far ahead of the USSR?

IIRC, in the 30s and 40s it was a lot more common and respectable to be a Communist due to the experience of the Great Depression an limited information about the failures of central planning.

Also, because many worker rights in the West were only very recently won after a very long and bloody political struggle by socialists, trade unions, etc (Who were bitterly opposed by the government.)

When said governments are very clearly acting against their own people, is it no surprise that defectors may choose to side with an opposing ideology?

Belief in progress, in general, and the URSS being the embodiment of it, in particular. E.g. "I have seen the future, and it works."

Preventing single-sided nuclear holocaust by creating mutually assured destruction.

Plans were drawn up and advocates were seriously proposing to enact them: https://www.businessinsider.com/the-pentagon-estimated-204-a...

I guess for the same reason a soldier puts his life at risk in a war. Patriotism, keep their family safe etc.

Service-for-school, money, personal safety (army is better than homelessness), fulfilling murderous desires (see current uproar over US Navy SEAL).

You used to be able to join the military instead of serving jail time. So not all reasons are the same.

They knew game theory and wanted to prevent another war.