The House of Rothschild vol 1 by Niall Ferguson

Basically, the Rothschilds moved from mercantile trading into finance, concentrating on sovereign debt. The conquests and aristocratic wars of the early 1800s taxed state treasuries, so that governments needed to place debt in order to fund their armies. This volatility created a lot of high return opportunities that the Rothschilds took advantage of with their good connections to kings' courts to get news of who was backing who (yay Europe).

Also, the teamwork of having five brothers across Western Europe created a news network that outcompeted all other news media for approximately twenty years in the early 1800s. That's a lot of beating up on lesser informed traders.

The only really effective plan would be a good property tax properly and judiciously levied to reach only the opulent and those who can afford to spare part of their income, but unfortunately it so happens that these very persons are the law-makers themselves and their patriotism does not go so far as to reach their pockets.

-- Abraham Montefiore, Chapter Four, 'A Court Always Leads to Something', The Economic Consequences of the Peace