Dominion War. He was given access because he fought major engagements in the war, I believe 8. He was a General before he came to power, before he freed the Remans.
Which would make sense when Shinzon was hated by the Romulans and left in the mines to die.
The meat of the story is whether Picard would be this man if he hadn't had his upbringing. Jean-Luc seems to think it's innate, his goodness. And, therefore, he can reach Shinzon.
And that amounted to what? The old, tired "I will kill you!" schtick. You would think that Picard's artificial heart would have given Picard a tangible example on how events of one's life can change how their entire perspective, but no. Picard actually says that Shinzon's heart is the same as his.
Shinzon is searching for his place in the galaxy.
Since Shinzon is the ruler of the Romulan and Reman Empire and controls one of the most powerful ships in Star Trek, I think his place in the galaxy is solidified. How much better can a man get than going from slave to ruler over your oppressors?
That's an old story, but one that's more intelligent than Generations' "I don't want to get old" story. Or "let's dominate the planet in the past" story of First Contact.
Except that there was more to First Contact than just "dominate the planet in the past". It was a tale of a Captain going to great lengths for vengeance against a foe that violated and used him in the past. It wasa also about a man who cannot see past his own selfish ambitions and what effect his actions will have on the future of humanity. And my personal favorite, a story about a man who long believed that humanity was so perfect that they would one day be like angels and gods and that their past selfs were so infantile and self-destructive, soon turned into something worse. And the real kicker? It took one of those people to show how infantile and self-destructive he had become.