Trajan was (I understand) the last conquering Roman emeperor (of the west). He conquered the area north of the Danube River, which is now Hungary, Romania, etc.
My question: there was still plent of europe to be had…why did the northern expansion of the Roman Empire cease with Trajan? Did the army break down?Or was the empire already too large to be controlled?
Anyway, did Trajan just announce to the people: “hey, we aren’t going to be expanding anymore, so send the army home”? -or something to that effect?
Territorial expansions were occasionally attempted after Trajan; Antoninus Pius’ wall (really more of a raised mound of earth) in Scotland extended territory beyond Hadrian’s wall, Marcus Aurelius’ famous column in Rome recounts his victory over the Marcomanni, and Septimus Severus spent a lot of time winning new lands in Mesopotamia. It shouldn’t surprise you that Roman emperors felt the need to demostrate their credentials for ruling the empire by conquering yet another barbaric tribe: Claudius had the Britons, Vespasian and Titus the Jews, and Trajan the Dacians (Julius Caesar, you’ll remember, used his conquest of the Gauls as a political tool to will the favor of the people back home).
This tradition didn’t disappear overnight, but soon met with the realities of managing a large empire. The main contrast comes with Trajan’s successor Hadrian, who adopted a deliberate non-expansion policy; this is usually why historians summarily declare that Rome reached its height with Trajan . Hadrian bought off warring tribes along the Danube, withdrews Roman legions in the East back behind the Euphrates, and built his world-famous wall. Speculation on this policy has filled volumes of historical research–you won’t find a simple answer to the question here–but certainly factors such as the administrative size of the empire (Hadrian’s gratest legacy, besides the eponymous wall, was a much-needed reorganization of the civil bureaucracy), the resulting rise in the power of provincial governors (by this time almost always a local rather than a true Roman), and the increasing prominence of provincial (non-Roman) leaders in the government that gave the empire a more cosmopolitan feel.
To think of Rome’s decline as a steady loss of territory up to the traditional fall-date of 476 would be a mistake, but there is no doubt it was taking greater and greater effort for the Romans to reclaim lost lands. To put it succinctly, the barbarian pressure never let up, but the economic resources required to keep a standing army ebbed over time (Edward Gibbons can tell you why:-). Civil wars in which one candidate for emperor tried to out-bribe the other for the loyalty of the army only accelerated the process.
Oooh, one other quick point…although you joke that Trajan told the government “OK we’re thru expanding now,” that is exactly the advice Augustus gave to the Senate when his will was read aloud (I think this is in Tacitus). His motives for this were likely selfish (“No one’s gonna rule a bigger empire than ME!”), but there it is.
See what happens when you get me started 
Also, as the empire got bigger, expansion became more difficult and less profitable. Expanding into Germany, even assuming the empire got around the psychological factors of the Teutenburg Wald, would give the empire what? Forests with barbarian villiages in them? There’s no money in it. Expanding east might have been profitable, (and the empire did try, mostly unsuccessfully, to expand into Mesopotamia), but to do that meant taking on the Parthians, who were as strong as the Romans.
Expansion into Germany was further deterred by the impact of Varus’ defeat in the Wald during the reign of Augustus, a disaster in which three legions were destroyed. It is difficult to underestimate the psychological impact this defeat had on future emperors (the legion numbers, for example, were pemanently retired).
Tacitus’ Germania, a kind of pamphlet detailing German geography, society, and customs written around 100 AD, is often cited as an example of Roman admiration for the barbaric Germans (similar to the ‘noble savage’ myth of the American Indian) in contrast to the growing decadence of Roman society. Whether true or not, the Romans clearly respected German military strength, and perhaps Tacitus is arguing this is a natural product of their purer culture (and a way to get the Romans back in shape!).
Another point to consider:
I read once that the borders of the Empire turned out to be contiguous with a map drawn of arable land around the Mediterranean. That is, farmland capable of growing food was scarce outside the borders.
Not only were there not many riches to be gained but, if you did conquer a new territory and wanted to station a a garrison there, it was really hard to feed them.
Because there’s no there there.
Prior to Teutoberger Wald, (noted by CJJ) Roman culture/government/civilization in Germany was working its way towards the Elbe and the Romans were fairly sure that the Germans saw the progress/Romanization as a good thing. After Varus’ disaster and all of the progress was lost the Romans more or less confined themselves to more profitable endeavors. Claudius conquered southern Britain.Under later emperors armies from what is now Scotland were destroyed, but the border never shifted much.
Dacia was kind of a special case - both a very wealthy kingdom and a perpetual thorn in the side of the empire, always threatening communications along the coast of the Adriatic. After Dacia was conquered there was a brief conquest of the spit of land between the Danube and Rhein to shorten the borders, but in the long run Dacia wasn’t worth the trouble of defending.
By this point Rome pretty much only expanded when necessary defensively.
Trajan also conquered Mesopotamia and probably would have liked to continue to expand eastward, but Hadrian and pretty much all of his successors (to the unified imperial throne) felt that it was not worth the trouble of expanding or even keeping troublemaking provinces outside the established empire. The pleasure was transitory, the position ridiculous.