There's a lot to unpack here, and I'll admit that I can't answer all of it, so let me just provide a little bit of insight. Having a claim to land in the Medieval, Renaissance, or Late Medieval period was absolutely a reason to go to war. However, it was by no means the only reason people went to war. In the 16th and 17th centuries wars were fought over trade routes, colonies, and especially religion. This time period is quite literally known as the age of religious wars. So by having a claim (legitimate or pseudo-legitimate) to land was a way to rally people around you. But you had to get people to recognize that claim.

I'd recommend the book I'm reading right now,

Fletcher, Catherine. The Beauty and the Terror: The Italian Renaissance and the Rise of the West. Oxford: Oxford University Press, 2019.

It discusses the Italian Wars, the sixteenth century conflicts between the Italian states, Spain, France, Switzerland, and Germany. One aspect of these conflicts that comes up again and again is that a significant factor in ruling and warfare between the Italian states (remember Italy does not become a unified nation until the 19th century) was receiving Papal recognition of your claim to the seat of power. Remember, at this time the Pope of the Catholic Church not only governed the Church itself, but also controlled the Papal States. He had an army at his disposal and everything. Receiving the pope's blessing essentially legitimized your right to rule, like when national leaders today recognize the winners of international elections.

Also, if you like the political, Game of Thrones-esque history, or learning about how the politics, socio-cultural environment, and art of the Renaissance period all connect, this book is phenomenal.

This seems like an excellent opportunity to diligently peruse the heydays of the House of Chilli, their meteoric rise in titles should fit in squarely with a posed question, how such a rise was perceived and coincidentally, their decline in 1456, again, due to the title. In this light, I will not focus on historical events per se, but rather the relations of the titles.

To disclose some notable land-markes, elevated to Counts in 1341,and finally the Princes of the Holy Roman Empire in 1436, and received from Sigismund of Luxemburg the independent principality, which title-wise made them equivalent to the Habsburg, thus between 1436 and 1443, there were disputes and small-time engagements between the two, which ended with the Chilli-Habsburg Treaty, where they legally lost the independent principality and remained princes in name only, an unparalleled event in the empire.

Hierarchy in HR ( Holy Roman Empire from now on )

Roughly, “Heerschildordnung” had seven levels, (1) Emperor, (2) Clerical princes, (3) Secular princes ( Division between them after the concordat of Worms ), (4) Lords ( (“Grafen” and “Edelfreien” ), and these four made up the aristocracy. Lower nobility (5)(6), “Ritter” and “Edelknechte”, respectively. Seventh were in a difficult spot, since they were limited to be vassals, and could not have their own.

Every level of the "Heerschild" had a corresponding form of address, used before the title and name. Traditionally, to the fourth level of “Heerschildordnung,” one would use edel or nobilis. But with late middle ages, there was further diversification. Lower levels of nobility, previously limited to “ritter” or “edelknecht”, started using the former addresses. So by the early fifteenth century, we get new forms in higher levels, “hochgeboren” and “wohlgeboren.” For princes, it was added into “hochgeboren fürst” or “illustris princeps". Also, with development of territorial lordship, princes started adopting dei gracia, which was formerly reserved to the Emperor.

With this in mind, Sigismund of Luxemburg elevated the Chilli to princes on 30 November, 1436, in Prague, and with this severed the link of vassalage between the Chilli and the Habsburg, his rivals.

…die durchleuchte furstyn frawn Barbaran romische vnd zu Vngernetc kunigyn des wolgeborn Hermans grauen zu Cilli etc seligentochter, vnsers lieben swehers vnd getruen zu vnserer gemahl geruchthaben ze nemen … nach abgang des wolgeborn Friedrichen grauenze Ortemburg … auch der egenant graff Vlrich an statt deshochgeboren Friedrichs grauen zu Cilli zu Ortenbgurg vnd indem Seger vnsers lieben swagers vnd fursten sun mit zwein aufgerakten banyeren,ains der grafschafft zu Cilli, das ander der grafschafft zu Ortenburg vnd Sternberg…

It was further ordered that they were to be properly addressed or be fined two hundred marks of gold for those who refused to address them properly by the title. So while from their archives we get significant confirmation, Habsburg did not address them properly until the treaty. A letter from Friedrick V of Habsburg ( or Friedrick III as Emperor since 1440 ) did not accept the title of princes. So, from thus period we have letters from the Duke Frederick Chilli, addressing them as “Edler und lieber getrewer von Cili", or simply “die von Cilli.” While in turn, cognizant of the inferior position, they presented themselves as “grauen zu Cili", while giving the Duke full respect.

Their position also worsened after the death of king Albert, when the war broke out between widowed Queen Elizabeth - daughter of Barbara of Cilli ( and implicitly her infant son Ladislaus V ) and Polish Wladislav III Jagiello, who was supported by majority of Hungarian nobility. In 1441, the Chilli pledged loyalty to the king, but in turn got recognized as vera atque legitima membra corone regni Hungarie.

After his death, and Ladislaus Posthumous as king, in 1446, the Chilli get a full recognition, not denied by any Hungarian house, cum illustribus principibus Friderico et Vlrico filio eiusdem dei gracia Cilie Ortenburge Zagorieque comitis necnon regni Sclauonie banis. And finally, in 1455, Ulrich of Chilli became informal guardian of Ladislaus V, and in 1456 the Hunyadi died, at the time being captain general of the realm, which got transferred to Ulrich.

In 1455, we also have the only recorded instance of Habsburg addressing them as “dem hochgeborn fürsten vnserm lieben öheim graf Vlreichen grauen zu Cili zu Ortemburg vnd im Seger etc, ban zu Dalmacien Croacien vnd in Winndischnlannden".

He was assassinated in November, 1456, holding the regency of Hungary for two months, by Ladislaus Hunyadi, and thus ending the Cilli lineage.

So, from this short period of 40 years, if we generalize, we get a war ( with long armistices in between ), plenty of devious politics, and an assassination that ended the lineage. While the titles might not be causes per se, but the benefits and everything they bring with them. The issues rarely escalated to war, as war is costly above all else, but were settled by higher authority ( rank ) or through various settlements, in this case, after the death of the last of Chilli, their possessions went to the Habsburgs, per the treaty signed.

Open for follow-ups.

Literature

Andrej Ivanuša: EKONOMIJA CELJSKIH GROFOV

Martina Bezovšek, 2012, Privilegiji mesta Celje do konca srednjega veka

Nataša Potočnik, 2016, Mesto Celje v drugi polovici 15. stoletja

Robert Kurelič, 2005, The Uncrowned Lion: Rank, Status, and Identity of the last Chilli

Nada Klaič, 1982, Zadnji Knezi Celjski v deželah sv. krone