For the first time in more than three years, the Federal Reserve has tightened its grip on the American economy, raising its benchmark interest rate to a range of 3.75 to 4 percent in a unanimous decision that reverses the accommodations of the recent past. The move reflects a sobering acknowledgment that inflation — fed by a prolonged Middle East conflict and an unprecedented surge in artificial intelligence spending — has proven more durable than policymakers once hoped. In raising rates, the Fed is asking the economy to slow its pulse so that prices might, in time, find their footing again.