In the short term the death of Geta weakened rather than strengthened the rule of Caracalla. The people were upset both by the crime of fratricide, and by the fear that the divine punishment for that crime might involve the rest of the empire as collateral damage. The army were upset, because the soldiers had sworn allegiance to both brothers. The senate, while not deeply disturbed by the death of Geta, was dismayed by the loss of so many of its members in the subsequent purge.
The flurry of legislative activity which followed the purge of Geta's supporters should be seen partly as a reaction to this unpopularity, and partly as the implementation of many of Caracalla's plans for the empire which Geta had blocked. For Caracalla, as for any other sane third century emperor, the first priority was to make sure that the army stayed loyal. This was accomplished in the first instance by straightforward bribery. Soldiers received a pay rise of about fifty percent across the board. There were also some reforms to the legal system which benefited the soldiery, but essentially Caracalla purchased their loyalty. Since this pay rise was considerably more than Rome could afford on a long-term basis, Caracalla's next two steps were primarily aimed at increasing state revenues.
First the silver content of the denarius (which had been slowly dropping for centuries) was more sharply cut; from around fifty - eight percent to fifty. Secondly a new coin called the antoninianus was issued. The coin had a face value of two denarii, but had a silver content of only 1.5 denarii. Debasement of the currency would provide temporary relief until inflation ate away the the gains. However, Caracalla had an altogether more ambitious plan for the long term. In 212 he issued the momentous Constitutio Antoniniana de Civitate, the declaration that all free citizens of the Roman empire were eligible for the Roman citizenship.
Exactly how the decree was implemented in practice is still debated by historians, but three consequences had great effect on the development of the empire. The first was that government revenues received a significant boost, because all the new citizens now paid taxes to the Roman state. (According to the historian Cassius Dio, who loathed and despised Caracalla, this was the sole reason for the decree.) Furthermore the standardization of the multiple tax systems which had existed heretofore briefly set back the ever-increasing bureaucracy with which the empire was becoming burdened, and this also increased revenues.
Secondly, the army received a short-term boost to recruitment because membership of the legions was open only to Roman citizens, and there were now many more of these available. However, service in the auxilia had for centuries been a route to obtaining the Roman citizenship, and since service in the auxilia was seen as less desirable than service in the legions, the auxilia increasingly became dependent on barbarians to make up for their manpower shortfall.
Finally, with his decree, Caracalla either knowingly or unwittingly completed the transformation of the Roman empire from being an empire ruled by Rome to an empire of Romans. This process had been going on for centuries, but henceforth most people of the empire saw themselves as one people engaged in a common project. The Antonine Decree was the main reason why the people of the Byzantine empire still called themselves 'Romans' almost a millennium after Rome itself had fallen.
To appease the people of Rome, Caracalla pushed forward the building of the magnificent baths commissioned by his father but which still bear his name today, although so ambitious was the entire project that the full completion required not only the remainder of Caracalla's reign but continued through that of his two successors before finally being achieved by Severus Alexander.
In his relations with the senate, Caracalla did rather as had Hadrian almost a century before. Also on poor terms with the senate, Hadrian simply had as little to do with its members as he possibly could. Caracalla took the same route. He left Rome in 213 and spent very little time in the city thereafter. Henceforth his career was that of a war leader.
Due to the very successful campaigns of his father, Septimius Severus, most of Rome's enemies were already weakened and subdued. This meant that Caracalla had to go actively looking for trouble. Caracalla started on the Rhine frontier, less because the Germans were causing problems than because it was important to be seen and acknowledged by the Rhineland legions. This was especially important as Caracalla had been quietly lowering the efficiency of these legions as he attempted to ensure that no provincial governor had command of more than two. This made the governors less of a threat as usurpers, but it also made the army less effective militarily. Thus the Rhineland campaign was mainly a public relations exercise by which Caracalla endeared himself to the troops by eating their rations, sleeping in the same conditions and publicly declaring that he lived only to serve the soldiery.
In actual military operations, Caracalla had noted that the Roman army was generally incapable of subduing the Germans for long. He also noted that rivalries and factionalism among the German tribes could be encouraged by giving judicious political and financial support to different Germanic elements. Basically, rather than using the Roman army for the job, it was more cost-effective to pay the Germans to beat themselves up. However, as with many of Caracalla's initiatives, under later Roman emperors this policy slowly degenerated. It became the harmful measure of bribing the Germanic tribes not to attack Rome; a policy which simply meant that when the Germans did attack they could afford the best armour and weapons available for the job.
However, this was not evident at the end of 213, when Caracalla could safely estimate that neither the Rhine legions and the German tribes posed any immediate threat to his rule. It was therefore safe to embark on a more ambitious project. Caracalla was an admirer of Alexander the Great, and he felt that the time was right to emulate the most notable achievement of his hero – the conquest of the east.