Well, if you place market orders on any combination of public exchanges for a total of 1.2m coins, the price of bitcoin would quickly approach infinity. As there are no where near that many coins for sale, all current sell orders would be immediately filled. Other sellers (and even long-term holders), seeing a chance to make a profit would rush to add new sell orders, quickly discovering that any price they ask for is satisfied.

So, you, know. If that's what you wanna do just make sure you have infinity dollars.

If you instead happen to be on good terms with Satoshi Nakamoto, you could maybe just buy (most of) the coins you want from him. * If he just gives you his private keys and pinkie swears not to use them himself, obviously the market wouldn't even notice and nothing would happen. * If he actually moves the coins to an address you own, this would be seen as huge news and the market would react both strategically and, let's be honest here, emotionally. The price would probably move sharply down first as people would be worried about a massive increase in supply on the exchanges.

Realistically, the best strategy to get that many coins with minimal slippage would be to accumulate them off-exchange, over time from individuals who deal in large amounts of bitcoin.

E.g. make friends with a bunch of miners, agree to long term contracts to consistently buy their coins at some fixed rate. The "fixed" part of that contract is important; you don't want your seller to just buy a bunch of coins on exchanges to hike up the price you're paying for each batch.

Anyway, the effect on the market would be to reduce the overall supply of coins, so the price would go up (supply / demand and all). How much it goes up would depend upon how fast you're buying as well as what sort of emotional reaction the market has to the price changes you've indirectly induced.

Good luck creating chaos!