The first significant increase in the federal income tax rates occurred during World War I. The U.S. government needed additional revenue to finance the war effort, leading to several changes in the tax system.

### Key Increases:

- **Revenue Act of 1916**: This act increased the basic tax rate from 1% to 2% on incomes over $3,000 for single filers and $4,000 for married couples. It also expanded the surtax brackets, with the highest rate reaching 15% on incomes over $2 million.

- **Revenue Act of 1917**: As the U.S. entered World War I, the need for revenue grew more urgent. This act further increased the basic rate to 2% and expanded the surtax rates. The surtax ranged from 1% on incomes over $5,000 to a maximum of 50% on incomes over $1 million.

- **Revenue Act of 1918**: This act was enacted towards the end of World War I and represented the most substantial increase during this period. The basic tax rate was raised to 6%, and the surtax rates were further expanded, with the highest rate reaching 77% on incomes over $1 million.

### Impact:

These increases reflected the government's need to fund the war and marked the beginning of a trend toward higher and more progressive income tax rates in the U.S., particularly during times of national crisis. The top marginal rates during this period were among the highest in U.S. history, and the income tax became a significant source of federal revenue.