The other option is that government can print money ex nihilo. The problem is that governments which do that have never been able to resist the temptation to completely debase their currency, resulting in hyperinflation. All Western governments have handed over the authority to create money to private central banks (America’s version of this is called the Federal Reserve). But the banks do not create money through printing presses. Instead, they create new money electronically every time they issue a loan for more money than they actually have on deposit. The reason they can get away with this is because only a small percentage of commerce takes place with actual physical money. The commercial banks in America end up creating $98 for every $1 held on deposit, which means that most of the money in circulation is actually debt money. Since every loan increases the money supply (a supply that is represented in bank ledgers rather than hard cash), it also depreciates the relative value of the money held by everyone else. Thus, central banking is also inflationary. However, the requirement to pay interest curtails the borrowing to a certain degree. On the other hand, when governments print money or mint debased coinage, as in Weimar Germany and the last three centuries of the Roman Empire, they tend to completely debase the currency in a shorter period of time than happens under a central banking system.
What this means is that if the American government wants more money to spend then it can raise through taxation, the only option is to go to the banks and ask for a loan.