Why battlecruisers traded protection for strategic speed

A force arriving after a crisis has been decided cannot compensate by being stronger on paper. Forward basing enough battleships could reduce that problem, but it would require substantially more ships and money. Battlecruisers offered another way to move heavy naval power quickly between distant commitments. A fully armored fast battleship became viable only when machinery had enough power density to combine speed with armor and adequate subdivision. Until then, protection required compromises, particularly in how much of the hull the armor covered. Thick armor over a restricted area was different from the fuller coverage associated with a fully armored battlecruiser.