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Would counterfeiting another nations currency on a massive scale and secretly putting it in circulation inflate their economy, if done enough?
Eventually. Here's the thing about inflation. Inflation is when a sort of random group of products (called the product basket) increase in price. So say you have an extra billion dollars put into an economy running a trillion in circulated cash (1% increase.) Say you give that cash to a bunch of people. Distribute through a laundering process of some kind. What's the natural occurrence? People buy more stuff. So these companies take in extra money. The company now says...hey, we're selling more of this stuff now, we have to increase the quantity of this item. Because of this extra money, they put more into making this item.
So you now have extra money in the system, resulting in a much easier credit system due to the fractional reserve system of banking. The effects of this can be seen today. Too much credit results in people making a lot of riskier investments than they should. This sets off a recession when the apparent well runs dry.
Also, a lot of money being dumped into the system makes it very difficult to plot correct quantity demanded for products causing price points to rise. This can also lead to a recession/depression on a large scale.
You'd see a lot more unemployment, a credit crunch, and a lot of product hoarding.