Every transaction that you make in Bitcoin is potentially a taxable event.
Let’s say you bought Bitcoin for $1,000 and after it went to $10,000 you buy a business class trip to Australia for $10k. When you pay the airline with one Bitcoin, you’ve just triggered a taxable event.
The IRS would say that you essentially sold your Bitcoin, have a $9k gain and used those proceeds to buy the ticket.
Which means you owe the IRS capital gains tax on $9k, which is 20% plus the Obamacare surcharge.
You can read the rest @
And here is another take on what is coming down the pike:
https://deutsche-wirtschafts-nachrichten.de/2017/12/03/us-regierung-erwaegt-kriminalisierung-von-bitcoin/ [translation required]
By the way, this is one of the reasons the powers that be want a "cashless" society - soon every online transaction will be traced, and taxed.
Sadly, I have to admit this is a good idea. Without revenue, nation-state economies soon will collapse. I'm 100 percent behind going after people who are trying to hide "their" wealth from their responsibility to the public good.