brief, inconclusive war break out between the two Germanic leaders. Roman retaliation Germanicus' campaign against the Germanic coalition Though the shock at the slaughter was enormous, the Romans immediately began a slow, systematic process of preparing for the reconquest of the country. In 14 AD, just after Augustus' death and the accession of his heir and stepson Tiberius, a massive raid was conducted by the new emperor's nephew Germanicus. He attacked the Marsi with the element of surprise. The Bructeri, Tubanti and Usipeti were roused by the attack and ambushed Germanicus on the way to his winter quarters, but were defeated with heavy losses. The next year was marked by two major campaigns and several smaller battles with a large army estimated at 55,000–70,000 men, backed by naval forces. In spring 15 AD, Legatus Caecina Severus invaded the Marsi a second time with about 25,000–30,000 men, causing great havoc. Meanwhile, Germanicus' troops had built a fort on Mount Taunus from where he marched with about 30,000–35,000 men against the Chatti. Many of the men fled across a river and dispersed themselves in the forests. Germanicus next marched on Mattium ("caput gentis", capital city) and burned it to the ground. After initial successful skirmishes in summer 15 AD, including the capture of Arminius' wife Thusnelda, the army visited the site of the first battle. According to Tacitus, they found heaps of bleached bones and severed skulls nailed to trees, which they buried, "...looking on all as kinsfolk and of their own blood...". At a location Tacitus calls the pontes longi ("long causeways"), in boggy lowlands somewhere near the Ems, Arminius' troops attacked the Romans. Arminius initially caught Germanicus' cavalry in a trap, inflicting minor casualties, but the Roman infantry reinforced the rout and checked them. The fighting lasted for two days, with neither side achieving a decisive victory. Germanicus' forces withdrew and returned to the Rhine. Under Germanicus, the Romans marched another army, along with allied Germanic auxiliaries, into Germania in 16 AD. He forced a crossing of the Weser near modern Minden, suffering some losses to a Germanic skirmishing force, and forced Arminius' army to stand in open battle at Idistaviso in the Battle of the Weser River. Germanicus' legions inflicted huge casualties on the Germanic armies while sustaining only minor losses. A final battle was fought at the Angrivarian Wall west of modern Hanover, repeating the pattern of high