Personally I joined too late to get a chance at getting to the top however from looking at the top nation:
25 nukes = 6250 NS (I'm assuming the top nation also carried sufficient spies to prevent them form being spied away, which should be easy cause the top nation made millions more than anyone)
I think second was under 10k NS, so the to nation would need only 3750 additional NS to have been first.
For example:
500 infra: 1500 NS, 200 tech 1000 NS, 30 level 5 planes: 750 NS, 300 land purcahsed: 450 NS, 4000 soldiers: 80 NS, 400 tanks: 50 NS
That equals 3830 NS. As you can see from above, those levels are low compared to anyone at the top (all roughly 1k infra)
Now I think the top nation ended with 1600 infra or something. Meaning the top nation would have to take about 7 nukes to be sent be knocked down to close to 500 infra.
The Winner finished I think with more than double the NS of the next closest competitor, and had 3 wars in the last week. Even if some think the first was "filled" (though clearly cleared by the moderation ) the other 2 attackers couldn't put a dent into the top nation. By the end, NO ONE was even in range to carry out any form of attack of the top nation. Even if there remained any nation taht could attack, it would basically require a highly ranked top end naiton (ie top ten) to sacrifice his/her nation to try to do some damage which would not be gaurenteed to eb successful.