Serious answer: In the US, capital gains and losses are defined relative to the US dollar.

The responses are valid as far as they go, but the larger point is that the government in a jurisdiction is free to inflate its own currency at great benefit to itself while simultaneously extracting taxes on fictitious "gains" of other currencies or equivalents (like BTC) against that currency.

Technically yes, the same rules apply as if you were trading in any international currency. If you sell yen at a profit to convert to USD & purchase something, you owe tax. What sort of tax can be complicated depending on how long the asset was held. I'm not an accountant, but I think assets held less than a year are taxed as personal income, so marginal rates apply. Assets held longer than a year are, I believe, taxed as regular capital gains.

I'm not sure what would/should happen with extremely short term trades, i.e, you convert to Euros from USD for a vacation, the USD crashes, you convert back upon return from your vacation. I doubt the IRS worries too much about it, but I suppose technically you should file Form 8949 along with a Schedule D.

you can and would declare capital losses on gold sold at a loss. Cash does not have gains/losses, as they are themselves the unit of accounting.

The inflation effect causing cash to lose value is something you suck up - it's incentive to invest the cash.

If you held any other foreign currency you would have to track it like this. For example, I live in Canada and if I bought USD and then spent it over the course of a year in Canada I would be responsible for the capital gain tracked on each expenditure based on the day's exchange rate. It's hard to see how Bitcoin should be any different.