Liberty was released in 1991 by the USDA's Oregon breeding program, a direct Hallertau Mittelfrüh derivative bred as part of the same domestic noble-hop substitution effort that produced Crystal and Mount Hood around the same period. Of the three, Liberty stays closest to its noble-hop parent's clean, restrained character rather than picking up much additional complexity from other lineage.
At 3-5% alpha acid, Liberty is one of the lower-alpha American hops available, well suited to the same restrained lager bittering role its German ancestor fills — a full-boil addition contributes a soft, clean bitterness without the sharper edge a higher-cohumulone hop would bring to the same style. Its aroma is described simply as mild spice, floral and clean, without a single dramatic note that distinguishes it sharply from Hallertau Mittelfrüh itself. Liberty's low alpha acid is a genuine feature of a lager recipe's typically low IBU target, not a limitation to compensate for — the IBU calculator will confirm its modest contribution lands right where a lager wants it.
Hallertau Mittelfrüh remains the closest and most obvious substitute, being Liberty's direct genetic parent, while Crystal and Mount Hood serve as the closest domestic siblings sharing the same noble-substitute breeding goal. German and American lager styles are where Liberty is used almost exclusively. Liberty's alpha range (3-5%) overlaps Mount Hood's (4-8%) rather than sitting cleanly below it — the two only diverge at the very bottom, where Liberty's 3% floor undercuts Mount Hood's 4% minimum, and at the top, where Mount Hood's 8% ceiling reaches well past anything Liberty offers. Liberty is still the gentler bittering choice of the domestic Triad on average, just not by a non-overlapping margin.