"Strike" is a more neutral or formal word than "attack". Saying that a country "strikes" another would be considered more neutral (not in favor of either side. Stating plain fact) and professional than saying that the country has "attacked" another. In general use, strike implies a single action against (a single blow, one bombing, one excursion to raid a place, ect.), while an attack might describe one offensive action, or a short campaign (Ex. "the attack on this city began with strategic raids, followed by a push towards the city center and parties sent in to place bombs and traps for the enemy).