Inflation in the United States held at 3.4% in August, a number modest enough to seem familiar yet consequential enough to move markets and reshape expectations. The Consumer Price Index, released Friday, came in above forecasts, driven largely by gasoline prices inflamed by geopolitical tensions — a reminder that the economy remains entangled with forces no central bank can fully govern. With the Federal Reserve's policy meeting days away, the question is no longer whether rates will rise, but whether higher borrowing costs can cool a fire whose fuel lies partly beyond American borders.