The Germanic groups crossed the River Rhine (the boundary of the empire) when it was frozen. The Roman had weakened this frontier because they needed to redeploy their troops in this area to Italy to fend off an invasion of Italy by an Ostrogoth king.
The Vandal, Sueves and Alans crossed the river Rhine in the winter, when it was frozen. This made the crossing easier. This river was the frontier of Gaul. In addition to this, its frontier garrisons were undermanned because many Roman troops had been redeployed to Italy the previous year to fend off a major invasion of Italy by Redegasius, an Ostrogoth king.
By that time, the Roman Empire already had much trouble keepings its enormously long borders adequately patrolled. These were the times that Roman Emperors followed each other in quick succession and most of them were generals of border legions that were proclaimed emperor after the fall of the previous one. Usually, the aspiring Emperor then took most of his troops elsewhere to make good his claim. Also, at the time most of the Roman border troops in Gaul consisted of non-Romans, many of them from Germanic tribes. They would drive back Germanic immigrants if under strict orders to do so, but would look the other way (undermanned as most legions already were) if left to their own devices. You also have to remember that there was not a strict division between the people of Gaul and the people of Germany: ever since the days of Caesar 400 years earlier Germanic people - often also the Roman army's Germanic veterans - had been settling in the border regions of Gaul.
The Franks.
The Romans considered them barbaric and when an alliance was tried it ended in the Germanic tribes ambushed and defeated three Roman legions in the Teutoburg Forest. The Romans were never able to conquer Germanic territories east of the Rhine river.
The Germanic tribes invaded the western part of the Roman Empire and caused it to fall. The eastern part of the Roman Empire was not affected by these invasions and continued to exist for another 1,000 years. The Germanic invasions were part of Migration Period. In this period many peoples north and east of the Roman Empire were migrating. Several Germanic peoples migrated from northern Europe to central Europe. There were also migrations by the Scythians (Iranian-speaking peoples) and the Huns from Asia into eastern Europe. This created a squeeze in central Europe and some Germanic peoples from that area (the Vandals, Sueves, Alans and Burgundians) migrated into Gaul and then also into Spain, and northwest Africa. These invasions were more than military actions. They were migrations which often involved the movement 100-150,000 people for each of these peoples. Moreover, the Germanic peoples had good soldiers and powerful cavalries. In its later days, the Roman Empire, had become overstretched militarily. Because of the mentioned migrations, its vast frontiers were under constant attack. This created a sort of cat and mouse game. The Romans had to gather large armies and deploy them to the areas under attack. This exposed other areas because many of its soldiers had been moved to the areas under attack. These areas were, in turn, also attacked. This created a pattern of raids into the parts empire followed by retreat before the Roman army was redeployed to reach the raiders. Over time this increased the strain on the Roman Army. The initial invasion into Gaul occurred at a time when Roman soldiers were withdrawn from Britain and the frontier of Gaul to fight an attempted invasion of Italy by an Ostrogoth king. The Romans defeated him, but at the price of the mentioned Germanic peoples taking advantage of this to cross the river Rhine and invade Gaul. Because this involved migrations, these invasions were no longer just raids. They were occupations of territories. The number of attackers and invaders was too large for the Roman army to deal with. When the Roman legions withdrew from Britain, the Angles, Saxons, Jutes and Frisians migrated there in waves and eventually took over this island. The Alemanni of southern Germany and the Franks of southern Holland and central Germany took advantage of the original invasion of Gaul to conquer territories there as well.
Several Germanic tribes invaded the western part of the Roman Empire to migrate in search of new lands to settle. The Romans were not able to repel these invasions. This started the process of the fall of this part of the empire. The eastern part of the Roman Empire was not affected by these invasions and continued to exist for nearly 1,000 years.
Gallienus ruled for 15 years (253-268), dealt with a number of Germanic tribes attempting to cross the Rhine and the Danube into Roman territory,he stationed troops in Milan, which was strategic because it allowed him to be able to quickly deploy troops to either the Rhine or the Danube, and he established a field army.
They should. Plants of the same species should be able to cross breed or cross pollinate.
Sheer numbers crossing coming in across borders so long as to be indefensible.
Gallienus ruled for 15 years (253-268), dealt with a number of Germanic tribes attempting to cross the Rhine and the Danube into Roman territory,he stationed troops in Milan, which was strategic because it allowed him to be able to quickly deploy troops to either the Rhine or the Danube, and he established a field army.
Yes. Atropine is a tertiary amine and is able to cross the BBB.