Infantry

INF are usually good for defense and sheer numbers. russia can use them for attacking germany by then numbers and against japan. being islands, UK/Japan can use them for defense. however, germany should use mostly air and tanks, and the US needs trannies to make good use of INF.

Yes, it is true.

Inf is the best cost effective unit for defense.
You need them in order to keep your more expensive units alive longer.
This is both true on defense as on offense

usual first turn purchases:
Russia w/ 24 ipcs - 8 inf
Germany w/ 32 ipcs - 9 inf, 1 arm
UK w/ 30 ipcs - usually a combination of AC and a transport, however may purcahse air or industrial complex.
Japan w/ 25 ipcs- 2 transports, 3 inf
US - w/ 36 ipcs - combo of trns and inf.

Round 2:
Japan, Russia, Germany - all inf, UK and US - inf and transports

Round 3: somewhat the same,
Round 4: some agressors may go a little crazy and buy more armor.

note: these are not hard and fast rules by any means. A lot of time purchases might include surprises such as IC’s, ftrs, bombers, subs, and rarely battleships. Armor, although quite useful at some points, are not a good standard unit for many reasons, including but not limited to those mentioned previously.