Consider NATO an Alliance and describe me why states form collective security?

NATO - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Nato)

I'd say it's so lots of little countries can put all their military resources together so if a bigger country does anything aggressive to one of them, they've got support!

It's the international equivalent of forming a gang! Bullies will leave you alone then, because 'my mates are harder than your mates!'

NATO - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Nato)

I'd say it's so lots of little countries can put all their military resources together so if a bigger country does anything aggressive to one of them, they've got support!

It's the international equivelant of forming a gang! Bullies will leave you alone then, cos 'my mates are harder than your mates!'

Thanks for your fast response. BUt when you see the NATO most of its members are not little nations they are big states like, USA, UK, France and so on. SO why these big states form collective security? I mentioned NATo because of this thing, it's not a combination of little countries.
Thanks and looing forward your fast answers.
Solman