Historically, cavalry were troops who fought from horseback. They were expensive to raise, train, and maintain, so they did not usually make up the main body of an army. Instead, the main body was composed of much less expensive infantry, and the more mobile cavalry acted to scout the enemy, screen one's own movements, and act as a shock force at crucial points in battle.
By the start of the Pacific War, horse cavalry were all but obsolete, as the horse was being replaced by the internal combustion engine. Both the Japanese and Russians maintained some horse cavalry on the Asian mainland, but these played no role in the Pacific. The 26 Cavalry Regiment was stationed in the Philippines when war broke out and became the last U.S. horse cavalry unit to see combat.
Armor was seen as the
natural successor to horse cavalry by many military experts, as it was
also
an expensive mobile force with great shock value. This led the British to develop what
amounted to two armor forces in parallel. Infantry support tanks were
viewed as mobile pillboxes whose role was direct support of the
infantry, while cruiser tanks were seen as the new cavalry. It was soon
discovered that light cruiser tanks were no match for German armor, and the distinction
between infantry support tanks and cruiser tanks faded as the latter
received heavier armor and armament at the expense of mobility.
Mechanization of the U.S. cavalry was well underway
when war broke out in Europe in 1939. Most of the remaining horse
cavalry regiments were converted to armored regiments in early 1942 and
reactivated as fully mechanized cavalry in 1943. By the time 1 Cavalry
Division arrived in Australia,
it was essentially an elite infantry division retaining the old square
structure. Other cavalry were fully mechanized and
organized into small groups whose role was exclusively reconnaissance.
Most of these were employed in Europe rather than the Pacific.
References
The Pacific War Online Encyclopedia (c) 2007 by Kent G. Budge. Index