Teutoburg, in which Publius Quinctilius Varus does not have what can only be described as a strategic stroke and has the good thought to send scouts out ahead of the main force when pushing through Germania. They spot the Germanic tribes, rush back and warn Publius who then withdraws to more even ground to reform and stand against the Germanic tribes. They win, due to better equipment and tactics and reinforced by spare legionnaires from a grateful Rome, continue the push into Germania and eventual subjugation of the nation. This leads to Roman suzerainty of all the lands right up to the Neman at its height, as well as the Eider. This has enormous ramifications for Roman culture, which absorbs much of the Germanic culture and language into their nation and leaves the Germans in Sycthia and Sarmatia as the dominant ethnicity, bolstered by refugees that don't wish to live under the aegis of Rome.
However, Rome still falls, and the migrations occur if in a somewhat delayed manner. Though as they essentially have a pincushion of Roman provinces between themselves and the migrants the Western Romans fare rather better, surviving not just in Iberia -especially around Tarraco- but Britannia as well. Migratory frustrations cause the Germans to push south more earnestly, and they lay into what would be the Byzantines and in fact completely evict the Greeks from their homeland, who flee to Libya, where they'll stay for the next 1000 years. The Slavs push in behind them, occupying most of Central-Eastern Europe as well as Finland. This leaves a vacuum that is eventually subsumed by Turkic peoples.
History happens, and here is the current world order. I could expand on this a bit, but I'm a bit huffy at the moment. Le sigh, such is real life. Enjoy.