No, he was not. There were some defeats of Macedonian armies or troops (against the Scythians f.e.), but then they were led by generals, not Alexandros himself. Some say however that he was responsible for the defeats because he gave the troops not the necessary backup, perhaps because of envy and distrust for some of the generals.

I think Mulceber and Macilrille told already the important facts concerning Caesar and the conquest of Gaul. Although I don't like this fellow I have to acknowledge his superb military and political abilities. He was one of the greatest antique generals. Please take into account that it was not "Rome against the Gauls", but for a certain degree a private war of a proconsul, backed only by a relatively small powerbase. A person with less aptitude and determination could have collapsed easily and Caesar was sometimes near defeat, although he tried to gloss over this.

"Centuries of civil war" in Gallia is misleading. There was no Gaulish state and no civil war, just war between the different tribes. Some tribes were divided internally and that was always the best chance for any foreign might, here for Caesar. That the Gauls were backwards in agriculture compared to Rome is totally new to me btw, it was more the other way round.

The Germanic tribes were in a similar situation as the Celts, only not so much developed, a bit poorer and more "barbaric". All in all not very sympathetic guys in my point of view, but that's personal. There was nothing like a "Germanic identity" or love between the tribes, but mostly war. Centuries of war so to say, before the Romans came. Germania became a Roman province (not officially but in fact) after harsh struggle about 4 BC and the Romans started to make it a part of the empire like Gallia, f.e. building civil towns. Why Rome did not try to really reconquer the province after 9 AD and 16 AD is a very complicated matter and has much to do with the personality of the leading persons, some coincidences and only some factual reasons.