The United States has emitted the most carbon dioxide historically since the Industrial Revolution. In cumulative fossil-fuel and cement emissions, the United States ranks ahead of China, Russia, and the United Kingdom because it industrialized early and maintained very high emissions over a long period.
This answer refers to total emissions accumulated across history, not emissions in a single recent year. China is the largest annual emitter in many current datasets, but its rapid industrial growth occurred much later than the United States' major expansion. The United Kingdom was an early leader during the Industrial Revolution, yet its cumulative total is now lower than that of the United States.
Historical rankings depend on the accounting method. Standard country comparisons usually count carbon dioxide from fossil-fuel combustion and cement production, while excluding land-use emissions and sometimes treating international shipping and aviation separately. If the question instead asked about per-person emissions, annual emissions, or all greenhouse gases, the ranking could change.
Cumulative emissions matter because carbon dioxide remains in the climate system for a long time and contributes to ongoing warming. The United States' historical position therefore differs from its current annual ranking, when it is generally behind China.