In May 2002 Vladimir Putin said Ukraine was entitled to decide on its own whether to join NATO and that he did not see such a decision as one that would “cloud” Russian-Ukrainian relations.
I think it's highly loaded to phrase this as NATO expansion. It sounds like NATO somehow coerced these countries into joining, or annexed them somehow. These countries came beating on NATO's door desperate to be let in. The instigating trigger for Putin's 2014 annexation of Crimea wasn't anything anyone in NATO said or did, it was the newly elected Ukrainian government's signalling that they wanted to join NATO.
That's not where I would put the emphasis. Though there was some courting, the anti-Russian sentiment in these countries was genuine.
Russia shares its borders with 14 other nations. What happened is that, one by one, many of these joined, or became partner to, an alliance which views Russia as something to be contained. Russian leaders may not have been worried at first, but that changed later.
Contained within it's borders. You mean like constrained from territorial expansion and annexation of neighbouring territory? Yes. Just as all the European countries are contained within their borders.
Being constrained within your borders should be the default for everyone. The fact that Moscow has a president that doesn't think that way is exactly the problem NATO was created to deal with.
I'm a realist, I know there are cases when military interventions are necessary. Preventing genocide in Kosovo, kicking the Iraqi army out of Kuwait. There can be messy situations where the right course of action isn't clear, but Ukraine is not one of those. Flat out annexing large swathes of neighbouring countries is precisely the problem NATO was created to counter. It turns out for very, very good reason.
Are we talking about 1999?
There have been... a lot of momentous geopolitical events since 1999.
That was long after the whole mess in Bosnia, so it's not like we hadn't seen a horrible bloody genocidal conflict that the US and European countries intervened in before.
Come now, two of the more plausible candidates for the beginning of this whole thing are the violated promise not to expand NATO Eastward and the American invasion of Serbia. Relations between America, and its allies, and Russia have been soured for decades now, with America being largely to blame.