Quietly, the Marines of Alpha Company, 2nd Tank Battalion, have joined the fight in Helmand province, Marine officials said. The unit replaced Delta Company, 1st Tanks, which became the first U.S. forces to operate tanks in the Afghanistan war earlier this year. Delta Company returned home to Twentynine Palms, Calif., late last month, but the mission rolls on. To date, not much has been shared publicly about 2nd Tanks’ mission. It is clear that they’re in northern Helmand province and attached to Regimental Combat Team 8, which oversees operations in Sangin, Musa Qala, Kajaki, Now Zad and other districts. In…