Pontifex means bridge. It means that the Pope is a bridge between man and G-d. The Pope is supposed to be Christ's representative on earth, i.e. "The Vicar of Christ." Anything he pronounces while sitting in the Papal throne, which is actually NOT in St. Peter's Basilica but rather in St. John Lateran, is 100% "truth." And all faithful Catholics must believe it.
This sort of concept does not exist in other branches of Christianity. Protestants do not worship Luther (thank G-d, he was an antisemite) or Calvin or any reformer. What they say is irrelevant unless it is scripturally based. Orthodox Patriarchs cannot introduce new doctrines at will - it comes from consensus and ecumenical councils. Only Catholicism invests almost total power in the hands of one sinful human being - the Pope. Such a Pope can call up a crusade, inquisition, war, etc. whenever he likes.
This is completely false.
The doctrine of infallibility you refer to has nothing to do with a physical chair.
The theology behind
ex cathedra is that God will not lead His church into error. Hence, the Pope cannot err when he teaches fundamental church doctrine. This has only been done twice in history, and in both cases they were fairly obscure points of theology (the doctrines of the Immaculate Conception and the Assumption of Mary). In both cases, it followed several centuries of theological discussion, there was overwhelming consensus among bishops and the general community, etc.
It does not mean that he is personally infallible, that he can't sin, that he can't make mistakes, that he can't make errors when teaching or writing, etc. It's a very rarely used doctrine and is typically blown completely out of proportion by anti-Catholic critics.
This doctrine has zero to do with any temporal powers of the Pope - your statement about calling up a war whenever he wishes.
BTW, you mention scripture in Christianity. Catholics believe that scripture is simply a snapshot of tradition taken at an early point in time. Catholics afford weight to both scripture, tradition, and theology. So does the Orthodox church. Protestants afford sole weight to scripture. It is a significant difference.
The historical artifact (the chair) you refer to does actually reside in St. Peter's - I've seen it myself. It's mounted on a wall
overlooking one of the altars. Medieval people believed it was the actual chair upon which St. Peter sat and taught, but no modern Catholic believes that to be actually true. St. John Lateran is the Pope's parish in Rome, when he acts in his capacity as Bishop of Rome.
BTW, regarding infallibility...there is actually something that is similar in Judaism. If there is a point of debate and there is intense study, research, and discussion by rabbinic scholars, God may actually adjust reality to make their conclusions true. I'm paraphrasing from an Ask JTF question from some time ago.