Between the wars against the Borg in 2367 and the Dominion in 2370-2375, Starfleet found itself short on both trained crew and starships. The difficulty expanded when the wars subsided; while the Dominion and Cardassia were in effect destroyed, the Romulan Empire still had a significant amount of power across the Neutral Zone. In addition, the reduction of forces throughout the Federation allowed an increase in illegal activities. While many settled worlds are capable of policing themselves, there are numerous worlds along the borders of the Federation that have struggled with the increase in crime.
Because of this, the 'unclaimed' space between the Federation and other political entities such as the Cardassian Union, the Tzenkethi Coalition, the Ferengi Alliance and the First Federation had to be brought to order. In addition, very little attention was paid to the space nicknamed the 'backside of the Federation', the area owned by the Federation opposing the Romulan and Klingon Empires.
As the Federation cannot truly justify continuous activity in these areas, Starfleet itself doesn't spend great amounts of time within them, nor does it often stray outside of the borders unless specifically needed or required. But since the days of early Federation incursions outside the Solar System, it has been realized that it does not do the Federation any good to simply allow these areas to be 'left in the blind'. Thus, in a bold, yet experimental move, the Federation created the 'Border Patrol'.
The Border Patrol is not an actual, official arm of the Federation; instead, they are a series of legal and systemic agreements between the Federation, Starfleet (as an auxiliary assistant), and a conglomeration of 'private contractors.’ There are many historical precedents of these types of organizations in many species throughout history; by example Earth used such organizations as Private Military Contractors, designated but non-aligned agents and even privateers. Many of the companies have been founded by former Starfleet personnel, as they are very familiar with this type of work. Reaching back into history, the concept of the 'Professional Military Company' was brought forward, and modified to suit the Articles of the Federation. As such, the individuals who came forward were given the position of “Captain-Owners” - each was given the opportunity to select one ship from the 'boneyard'. The ships selected had been decommissioned and placed in holding for parts and training.
Once each Captain-Owner had his, her or its ship, they were sent out to recruit their own crews. Some Captain-Owners selected their crews based on the strict profiles presented by the Federation, while others were much more 'liberal' in their options. Regardless of the method in which each ship was crewed, its Captain-Owner was assigned a patrol area, and a contract to carry out.
Many of the individuals within the Border Patrol are former Starfleet officers who have left the service of the ‘Fleet, but still have eligibility to work within a contracted environment. Other former ‘Fleet personnel, dismissed for less than stellar conduct or actions, can be hired on a case-by-case basis, with approval from Starfleet itself. The only other requirement in hiring personnel into a Border Patrol company is that the individual must either be a Federation citizen, or a recognized citizen of a Federation ally (for example, the Klingon Empire).
Border Patrol missions always include the security and observation of the borders of Federation space. However, FBP ships also carry out other missions, to include transportation of goods and people, tracking and apprehending criminals or illegal organizations, passive and active intelligence retrieval, and whatever other missions the Captain-Owner is contracted to or agrees to honor.