So I guess we have to start at the first distinguishable "Sundanese" polity and that is Tarumanagara. Historical sources are limited but by most accounts it was probably the most powerful poilty on the island of Java and so didn't face any threats by Javanese powers, chiefly Kalingga to its East. It's worth noting too that, at this time, the distinction between Sundanese and Javanese cultures were far less distinct than now (where I guess we can say that the beginnings of coherent and distinct Sundanese and Javanese cultures came from Pajajaran and Majapahit respectively, ), and so even if Tarumanagara were to be subjugated by Kalingga or another power in what we now know to be culturally Javanese lands, the distinction between Bhumi Sunda and Bhumi Jawa had not yet existed (bhumi = land).

Tarumanagara did however was on the receiving end of Sriwijayan incursions, which more than likely led to its decline and forced it to move its seat of power inland, after which more familiar and prominent historical events were to transpire. Fast forward to around the 14th century, the Sunda Empire and Javanese finally came to a head, which culminated in what is known as the Bubat "War" as you may know, but in actuality was more akin to an ambush leading to a massacre of the Sundanese royal family and outnumbered retinue. This is quite a well-published and told event in Sunda-Java history so I need not recount it.

However what is not spoken about much in the literature, to my knowledge, is why Gajah Madah opted for subterfuge rather than military conquest (as with Bali). Perhaps the most obvious reason is that Sunda at the time had a technologically comparable military, a formidable navy and a territorial advantage due to being boundaried by the Parahyangan highlands. It's also worth mentioning that the by this time the Sundanese had developed a reputation for having strong magic and Parahyangan itself translates to "The land of the ancestors", a boundary that the Javanese respected as Sundanese and in later years after the Muslim polities came to power would come to be land which they were reluctant to tread for this reason. This might sound somewhat absurd, but magic and the invisible world formed a huge part of these people's world views and how power was exerted. But I digress.

The ensuing massacre deeply shamed Hayam Wuruk and was viewed with contempt and disgust by the Majapahit court. It led to the end of Gajah Mada's career and he essentially faded into obscurity. As for Sunda, it caused them to forever distrust the Javanese and break all ties. Gajah Mada and the Bubat incident were the only real or significant attempts to subjugate Sunda up until this point in history. Prior to that, due to Sunda's power, no attempts were made by Javanese polities, and Sundanese foreign policy did not contain any expansionary or conquesting motives which might have invited other polities to bring them to heel. It wasn't until the rise of Muslim polities that Javanese-cultured powers sought to subjugate Sunda. Demak eventually sacked Pajajaran by the late 16th century, by which time the Sundanese polity was already in disarray after losing Sunda Kelapa to Demak and be under the leadership of a succession of incompetent leaders. The Sunda Kingdom didn't crown a a successor after Suryakancana. The legitimacy of power in Javanese polities was often symbolised by objects which hosted otherworldly power (related to my comment on magic and spiritual beliefs before). If the monarch no longer has these objects, then their legitimacy goes with it. As for the Sunda kingdom, the sacred stone slab on which their kings are consecrated was moved to Demak after being sacked. The Sundanese aristocracy moved other items of significance "called "pusaka", which can range from heirloom edged weapons to chariots, items of clothing etc.", to the city of Sumedang Larang, which then saw itself as the rightful successor to the Sunda Kingdom and claimed itself an independent polity thereafter, beginning a new dynasty starting with Geusan Ulun.

For this reason Sumedang became quite significant in Sundanese history among Sundanese people, but a relative blip in the overall history of the power struggles on the island of Java. Sumedang remained independent until it became the vassal of Mataram in 1620, without bloodshed.