"By 1908, the time was right for a new kind of agency to protect America. The United States was, well, united, with its borders stretching from coast to coast and only two landlocked states left to officially join the union. Inventions like the telephone, the telegraph, and the railroad had seemed to shrink its vast distances even as the country had spread west. . . . America was . . . a new world power on the block, thanks to its naval victory over Spain."
From an FBI official history
There's a certain serendipity in President Donald Trump's precipitation of crisis at the Federal Bureau of investigation on the 100th anniversary of the United States' entry into World War I.
America's federal law-enforcement and intelligence agencies, of which the FBI (or its precursors) was among the first, took shape in the 20th century. Their rise is inseparable from that period's dominant themes: national growth, international extension and, comparatively speaking, political consensus.
Certainly, the bitter partisanship of the 19th century, culminating in the Civil War and Reconstruction, helped prevent the development of an American counterpart to, say, the centralized detective force that Joseph Fouché built in France. No party in Congress trusted the other sufficiently to create a secret, or semi- secret, agency that might later be turned against it.
In the 20th century, Republicans and Democrats didn't agree on much, but more often than not, they shared a sense of core national interests, and, following from that, a sense of who federal detectives and spies might appropriately target (even when that consensus led to excesses, such as CIA domestic surveillance and the FBI's campaign against the Rev. Martin Luther King Jr.).
To the extent it guided their oversight of intelligence agencies, that baseline consensus overcame the politicians' qualms about the undeniable tension between democratic norms on the one hand and the dark arts of surveillance on the other.
As Trump's bold decapitation of the FBI, and the reaction to it, illustrate, however, this massive agency - 14,000 special agents, an $8.7 billion annual budget - has outlasted the national consensus that gave it life, and upon which its legitimacy ultimately rested.