Xibei San Ma
Latvia's strategy is based on a defensive doctrine. Her entire military should focus on just defending Latvia against the Soviet invasion in 1940. One should also strive to ally with other Baltic countries and Finland when that becomes possible or even ally with Germany, though that may mess up her invasion of France and the Low Countries. If the former allies are chosen instead of Germany, be sure to keep good relations with Germany in order to avoid being invaded by them.
Latvia starts the game already allied with Estonia, so should try and trade blueprints with each other as often as possible to boost the chance of surviving.
It is unlikely the other two Baltic states (unless human controlled) will do anything other than bow to Soviet pressure starting September 1939, so you cannot count on them to buy you any time at all.