Nehru had the golden chance to secure India's place for a century. Here's I believe the most horrible unforgivable mistakes we are still paying for.
1. Allowing Pakistan to happen and not going to war immediately on them the moment the British left. Balochistan, Sindh, Kashmir etc. None of them would have been part of Pakis if Nehru had acted. Jinnah was already old and probably had months to leave. This period would have ensured a landlocked Pakistan controlling only their punjab region. A landlocked Pakistan with no access to see would have been a smaller Nepal.
2. Not anticipating Mao's victory and planning to use the 3 years from 1947 to 1950 to take Tibet into India's protection, fortify defenses and building up armed forces. when the Chinese were busy with their civil war and there was No Mao and his band of shitty commies. If we had Tibet as a protectorate and had repelled the Chinese aggression towards Tibet, China would have never been the power it is today.
If Nehru was actually street smart and not a Mahan intellectual, India would have been in a much safer and much powerful position. It would have changed the destiny of our country.
I am no fan of Nehru ( i am a die-hard Bose-ist) but i think point #1 is overblown to a large degree.
One thing i was once told, by a person who interacted WITH major congressi types during the 40s - was that people often forget, India didnt really have an army till late 1950s.
Because the army that India inherited, were all filled with British officers or total ghulaams of the british who would easily go against high command if it suited them. Remember, the likes of Manekshaw were mid ranked officers at 1947 and Nehru/Indian govt had to spend years curating the forces.
For eg, if you look closely into the first Kashmir war, we see that the Pakistan side of action is run virtually completely by actual British generals/majors.
And their reason for gobbling kashmir was to serve british interests - they wanted to use Kashmir as observation point for Chinese activities in Lop Nur, where Chinese were persuing their nuclear ambitions/going to persue their nuclear ambitions as it was clear by late 1940s that the commies WILL try to get a nuke as well and China was no exeption.
Which for Brit guy makes sense-they saw Pakistan as more likely to stand up to communism than the highly left leaning indian congress so picked them.
Nehru was a guy who a case like Cercei Lannister: smart but not that smart yet thought of himself as super smart.
He did a lot of harm in the 50s and 60s IMO, particularly re: China but in terms of immediately doing anything in 1948-1955 period, that would've been a fools mission and i am glad Nehru did pruning of military brass before getting any sort of military adventure ideas in his head.
2. As for interfering in Tibet - he couldnt have, because Tibet in 1947-1952 was stuck in idiot Dalai Llama tibet mode, where they believed their best interest is to be hostile to India and friendly to Beijing, coz then power in India wont try to eat it and Beijing will send one envoy who will just write reports to beijing while you run tibet as dalai llama and his cohort like you have for last 300 years.
People overlook this point - that Tibet didnt WANT our help till China came guns blazing in 1952, till then Tibet was the good old tibet of 'fuck you india, i am china vassal...DADDY CHINAAAAAAAAAAAA' mode.