In Game of Thrones, Robb Stark’s desire for true love caused him to rebuff an arranged marriage and ultimately led to his downfall - are there any real life examples of a ruler choosing “love” over politics / duty? How did it play out?

I haven't read or seen Game of Thrones, but I know it references the Wars of the Roses. It's possible that, with Rob Stark's marriage, Martin had Edward IV in the back of his mind.

Edward IV became king of England in 1461, amid the galactically tangled mess of the Wars of the Roses - which, boiled down to its simplest explanation, was a feud between the houses of York and Lancaster for the English throne. Edward was from the House of York.

His position on the throne was far from secure, though. The Lancastrians showed no signs of giving up their pursuit of the throne - and an awful lot of English nobles were on the Lancastrian side. Edward needed an alliance with someone very powerful, to strengthen not only his own position on the throne but also England's position internationally. One important way to form alliances was via marriage. His mother and his closest supporter, Richard Neville, Earl of Warwick, were putting a lot of work and negotiation into arranging a marriage for him with either the daughter or the sister-in-law of Louis of France.

And then in 1464, Edward secretly married Elizabeth Woodville, the daughter of a minor Lancastrian knight and the widow of another Lancastrian. She was by all accounts stunningly beautiful, and the general assumption is that he married her either for love or for lust - the story is that she refused to sleep with him without marriage. He clearly knew this was not going to go down well: he kept the marriage secret until Warwick's negotiations were advanced enough that Edward was forced to say 'Um, guys, about that marriage thing...'

Edward's supporters were, unsurprisingly, not pleased. His Privy Council informed him furiously that 'she was no wife for a prince such as himself, for she was not the daughter of a duke or earl'. This wasn't just snobbery: the point was that Edward had thrown away a hugely valuable opportunity to form an alliance that would benefit both his house and the whole country, just because some random woman was cute. Very much an example of the 'love over duty' that you're asking about.

As for how it played out... On a personal level, as far as we can tell, the marriage went fine. They had ten kids together, and while Edward did have mistresses and illegitimate children, the marriage seems to have been happy as far as we can tell. Edward definitely listened to Elizabeth when it came to making decisions at court. This is where the problems kicked in.

Elizabeth had a dozen siblings and about a million relatives, and she was very serious about getting as many as possible of them into advantageous marriages and positions of power. And Edward went with it. This may possibly have had a political function for him - these people had all been Lancastrians, so it sent the message that he wanted to build cross-party bonds and establish unity - but it did not play well with his family and long-term supporters. They had fought the Lancastrians for Edward, they had been expecting to be rewarded for their support now that he was on the throne, and instead they got to watch him give random low-level Lancastrians the marriages and positions that they had wanted for themselves and their heirs. It's as if Joe Biden started handing over all the most coveted positions in his administration to random members of the Ted Cruz family: he would find himself surrounded by a lot of very pissed-off Democrats.

Warwick, understandably, was especially angry about this - even more so when Elizabeth's father, the newly created Lord Rivers, started replacing him as Edward's most influential counsellor. He and Edward's brother George - heir to the throne at this point, since Edward didn't have a son yet - even ended up leading a rebellion that drove Edward temporarily off the throne and into exile, to a large extent because they had had enough of the bloody Woodvilles taking over everything. It didn't work: Edward ended up back on the throne, and the Woodvilles continued the process of basically annexing everything they could see.

So, on a political level, it didn't go so well. The marriage didn't lead to Edward's downfall - almost, but not quite - but it destabilised his entire regime, and its ramifications went on even after his death in 1483.