Why Fight?

The problem explained
A first thought may be that fighting in the antique world was inevitable, like in ours. But human population still was extremely thin and there still were enormous amounts of unused fertile grounds. Farming was an option to everyone. Yet, and this may be a surprise from that observation, many chose to become a soldier and to aspire the status of war lord. The explanation is that if you are strong, fighting and pillaging an easier way of life than farming. So the strong  are naturally tempted to opt for the easy life of war lordship. The story below illustrates this initial drive triggers an self enforcing power: once you are maintaining an army in the way this was done in antiquity, the continuous search for new enemies and the attack on them was the only way for army leaders to fulfill the needs of their armies and thus survive, and stay in control of their troops. In other words, war lords tend to become the slaves of their constantly hungry and needy armies, even in a much more compelling way as parents become the slaves of their hungry and needy children.

After having suppressed the rebellion in the Rhine legions of the Roman Empire (14 AD), Germanicus still had to cope with lots of anger and frustration in the army.  By way of outlet of that, he set out to fight against the Germans. Not just a little bit along the Rhine, but deep towards the Northeast. Until the Teutoburger forest (near where you now find Bielefeld).
That  was the place where "Arminius", the German general he was fighting, had destroyed three complete Roman legions three years earlier.
The Teutoburger forest is deep, and far. You even have to cross the upper course of the next river, the Ems. Why did he do that?
Where Tacitus describes the diplomatic rhetoric of the German leader Arminius ("Hermann" properly pronounced), one of the subjects is tribute. Tributes form the Germans might be something interesting for Germanicus to acquire. But Germanicus does not in the least show such an interest: the Germanic tribe called the "Chats" were "so surprised by the advent of Germanicus that everybody who, for reasons of age or sex could not defend him or herself was grabbed and killed immediately. The men who could defend themselves had swum over the Eder...[a tributary to the Ems]".
Such a type of attack does not result in profitable taxation. It makes desperado's out of groups of surviving strong young men whose families are destroyed. So it seems Germanicus was more engaged with fight therapy and spoils to wipe the emotions of the rebellion out of the souls of his soldiers. And those German gangs of young muscular desperado's could, after getting confronted with the dark sides of raiding their own rival German tribes, and go for Celtic women across the Rhine, be expected to be smart enough to see the option of registering for Germanicus' army.
Then, Germanicus' legions arrived in the Teutoburger forests, where he knew they would find the camps of the destroyed legions, quite useful to redirect the minds of his rebellious soldiers to the "real enemy of Rome".
In the area between the last two camps -the last one was a lot smaller than the one before- "the bleached bones were lying all over or on heaps, depending on whether they had been fleeing or kept order. Next to it, the broken spears and the skeletons of the horses were lying, skulls were nails to trees. In open areas in the vicinity they found macabre altars on which the Germans had slaughtered the staff officers and the centurions".
The armed disaster tourists "buried the Roman soldiers there, six years after the catastrophe...they were all relatives to them, people to whom they felt connected through ties of blood. While they were doing so, they got more and more embittered about the enemy...".
And that seems to be the main aim of this bizarre campaign by Germanicus. Germanicus' exercise bears close resemblance to what other leaders do when cornered by there own people: turn attention to an enemy. A contemporary example is the occupation in 1982 of the Falkland Islands by the then cornered government of the Argentinean president Lieutenant General Leopoldo Galtieri. This considerable enhanced his popular backing until he lost, and it considerably enhanced the popular backing of British prime minister Margaret Thatcher mainly before, but also after she won.

Back to Germany 14 AD. The subsequent battles between Germanicus and Hermann remain undecided, and Germanicus feels he did enough to sail back up the Rhine with his relieved and purified  legions.

Germanicus' cavalry on the Dutch Wadden shallows
A part of the cavalry, however, got orders to proceed North to explore the coast from the mouth of the Ems to the Rhine.
Modern Dutch readers will already understand that this was going to be a disaster: this is an area with vast low lands covered twice a day with sea water by the tides: the mud-flats of the Dutch Wadden shallows. Romans knew nothing about tides.
Tacitus: "the area became inundated: the sea, the beach, the land, everything looked the same. It was impossible to see where it was safe and where you had firm ground under your feet, where it would be deep and where not. They were thrown over by the waves, pulled down by whirlpools; pack-animals, luggage, dead bodies floated around and came in collision with the soldiers. Of some divisions  the ranks got completely in disorder".
That last statement must have been by far the most disturbing for Tacitus' contemporaries: dying is bad, but dying out of rank must have been the ultimate nightmare for a citizen of the Rome.
Tacitus: "One moment the water would stand to their chests, a moment later to their mouths, sometimes they lost the ground under their feet and got swallowed by the water".
Hermann, knowing the trade of travelling on the Wadden shallows, lured part of the cavalry back over a swampy road on which they got stuck and killed. The army leaves its equipment in disorder and flees, horses slipping on the mud and their own blood.