ATHENS — Prime Minister Mitsotakis was hardly exaggerating when he told the Greek parliament on Monday that “for the next few weeks, we must be on constant alert. We are at war.” As firefighters battle devastating wildfires that have scorched areas near Athens and the islands of Rhodes and Corfu, forcing an unprecedented mass evacuation of tourists by land and by sea, the outlook for the rest of a summer already marred by searing heat is uncertain at best.