Japan never had a chance, while the Goa'uld at that point could easily beat Earth - remember: The Tau'ri (Earth) has a snowball's chance in hell against even one mothership (them blowing the two Apophis brought was pure dumb luck and a chance of one in trillions probably (they knew the gate address and gated onto the ship, which had no gate defenses because Apophis thought that nobody knew that gate address!))!
Nukes (even with Naquadah) don't work and there are no other weapons capable of reaching or damaging the ship
Even during seasons 5, 6, 7 and 8 they really didn't stand a chance against even one Ha'tak if you took the drone-chair out (which the Wraith later on DID and it left Earth defenseless!)...the Prometheus couldn't damage a Ha'tak and neither can a BC-304 without APBWs!
The Eurondan stuff would equalize the playing-field a little (who knows what they can offer? Can those Aerofighters go into space? Can they damage a Goa'uld ship? Do they have other weapons that might work (like a large version of their energy-based hand weapons)?)...so what Jack and Daniel do is way more immoral than working with Space-Nazis!
They could have condemned the population of Earth to DEATH (a population they kept the secret of there being aliens that wanted them either wiped out or enslaved from! Seriously, I'd be pissed if any government did that and I would demand the heads of those leaders if something like this got out (and especially if I learned that the SGC let tons of promising technology slip through their fingers, like those Androids that Harlan made, the Eurondan tech etc. etc. - thus endangering Earth by putting their morals first instead of survival!))!
Doing that because you are angry and feel slighted/cheated/betrayed? Damned, such a person is not fit for command! Seriously, Jack should have received some form of punishment and the rest, too!)
Don't get me wrong, I like the team - but sometimes they act like school-children instead of professionals! Hell, they didn't even try for say a Tolan black market (I doubt that there aren't Tolan willing to sell some pieces of tech for something Earth can provide, be it materials for a researcher who didn't get a government grant (they still seem to have a capitalist society, with some elements of socialism! So you can problably still buy people!)) etc.!
Not to mention that the SGC never established an intelligence section (they were totally dependent on the stingy and often dishonest Tok'ra -.-)
Just to be clear, the choice the team made in the end was a moral and ethical one. It didn’t make them immoral for NOT siding with Nazis even if it would save Earth. It IS a tactically unsound idea. But as we found out it all worked out ok.