The marriage was of course not the key nor the core of the political alliances roman leading aristocrats concluded.
There are several examples of political alliances sealed by marriages that ended in political enmity with or without divorce.
Scipio Aemilianus and his wife Sempronia hated each other and Scipio was a political opponent of Gaius Gracchus and the populares at least from 133 on. Some historians have dated the political drift between Scipio and the Gracchi as early as 142, because the Gracchi had historically been allies of the Claudii Pulchri before the marriage between Tiberius Gracchus the father and Cornelia Scipionis, one of the two real daughters of Africanus.
However, Scipio Aemilianus and Sempronia never divorced.
Pompey's optimate friends already urges him divorcing from Julia as early as 57.
But I disagree with slydesertfox on the fact that Pompey ended his alliance with Caesar because it was supposedly killing politically.
Pompey gaines much from the Triumvirate although he gained less than Caesar in proportions.
The Triumvirate served Pompey's goals.
First of all It saved Pompey from the risk of political extinction he faced in 61/60 when most of the political stage was banding against him.
Then, in 56/55, Pompey understood that the Triumvirate remained his best option and that all the optimates would bring his own downfall the moment after he dropped Caesar if he ever stopped Caesar. The fact is that Caesar was a lawful ally. And that's certainly the main reason for the reconduction of the Triumvirate, a very political decision in which the real love Pompey felt for Julia was certainly marginal.
The Triumvirate finally ended because Pompey decided that he could audition his support with Caesar's enemies in order to strengthen his own position. Cicéron had perfectly understood that Pompey acted this way because he could not stand the idea that Caesar would become his equal on the political stage thanks to the prestige of his gallic conquests and his popularis stance. Pompey miscalculated and pushed Caesar in a corner, committing this way the same mistake ad Marius committed against Sulla when he had Sulpicius Rufus deprive him of his asian proconsulate.
Caesar was really ready to negotiate and to compromise but he would never accept humiliation and political extinction. That was the cause of the civil war : the declining First man in Rome (Marius in 88, Pompey in 50) who outs a terrible mess and provokes a chaîne reaction because he miscalculates in order to retain the lead on the political stage.
Now, if Julia had lived and her son lived, It would not have prevented a breach with Caesar, nor maybe even the civil war.
The son of Pompey and Julia's destiny was probably to be adopted by his material grandfather. Pompey already had 2 adult and healthy sons by Mucia : he did not need a third son keeping his name.
But It would certainly dramatically change the world's fate because Caesar would never have adopted Octavian if he had had an adoptive son, his real grandson, bearing the name of Gaius Julius Caesar.
Hindsight is very deceptive. However intelligent Octavian was as a teenager, nobody, absolutely nobody (including Caesar) could guess how extraordinary was his potential, nor how he would reach an extraordinary position of power. The young man had a poor health. He even was literally a coward and a military non-entity. There were other brilliant teenagers in the roman nobility.
Besides, I have already explained (based on several historians among whom Luciano Canfora) on this forum that Octavian certainly was a temporary heir, and not the first one.
Caesar's first known heir was Pompey from 59 to 49/48 (cf. Caesar's will which he made public during the civil War.
Then, Caesar's secret heir probably was his cousin Sextus Julius Caesar, the grand son of the consulship of 91 who was Caesar's uncle. This Sextus Julius Caesar was born around 78 and he was murdered in 46 in Syria where he was the caesarian Governors. And guess what, 46 is precisely the date when Octavian appears on the political stage.
Augustan propaganda has tarnished his memory probably because Octavian had to destry the idea that there could have been another heir and that he was not predestined by the Gods to rule the roman empire.
