I would like to start out by talking about Sir Robert Peel. He was known as the Father of Modern Policing. Sir Robert Peel created the London Metropolitan Police Force in 1829.  He and his commissioners created a set of Nine Policing Principles which are as widely crucial and urgent today as they were two centuries ago. These are still relevant today. By creating these 9 crucial policing principles he and his crew established a rock solid foundation set of guidelines for policing. Sir Robert Peel talked about the idea of being proactive essentially eliminating crime before it happens. He talked about doing this by drumming up public support for the police force. He thought that reactive policing was not the way to go. He thought that preventative policing with more effective. He got rid on the citizen watchmen and created a paid position known as “bobbies”.
The Political Era
The political era occurred from 1840 to 1930. This occurred due to the development of industrial cities. During this time there was a growth in population in these cities which meant that there was a growth in crime. Prohibition also caused an increase in crime. During this time President Herbert Hoover created the Wickersham Commission. This was a set of reports on the criminal justice system and law enforcement agencies. During this time there was a rise in organized crime. This was said to lead to police corruption.
(Eras in American Policing – e-Roll Call Magazine andragogytheory.com/2016/01/05/eras-in-american-policing/)
 “ Intrusive searches for alcohol during Prohibition destroyed middle-class Americans’ faith in police and ushered in a new basis for controlling police conduct. State courts in the 1920s began to exclude perfectly reliable evidence obtained in an illegal search. Then, as Prohibition drew to a close, a presidential commission awakened the public to torture in interrogation rooms, prompting courts to exclude coerced confessions irrespective of whether the technique had produced a reliable statement.”  (Wesley M. Oliver is Professor of Law at Duquesne University)

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  Though intrusive searches caused much distrust in the police new laws were generated to create better guidelines for policing.  As these new laws and guidelines were created the public slowly regained more trust in the police.  This has impacted policing today by the abilities of the police to do their job in a more effective manner, and still protect citizens rights and trust.
The Professional Era
“The professional era rejected politics as the basis of police legitimacy. The civil service system was implemented which ended political influences in the hiring and firing of officers. Police were no longer seen as working for the political leaders and seen as “law enforcement”.”
(http://www.communitypolicing.com/history-of-policing)
This era also produced new concepts such as professional police training and police academic institutions.  Many police were removed from foot patrol and introduced into patrol cars with radio communication capabilities. “Thus showing mobility, power, conspicuous presence, professional distance from citizens and control of officers.  “(http://www.communitypolicing.com/history-of-policing)
Police Crisis in the 1960’s
  In the 1960’s turbulence was created by a variety of legal and social influences who had renewed interest in police reform. Police were seen as racist, denying equal justice to black communities. The Civil Rights Movement was also very big at this time. Police were also known to brutally abuse power and racial bias using deadly force. There were also several influential Supreme Court cases that took place during this era. For example Miranda V. Arizona. The court ruled that a suspect must be advised of their rights before being interrogated.
“The Warren Court handed down -`a series of decisions between 1961 and 1969 that expanded citizens’ due process rights and limited police powers. Some police administrators complained that Fourth and Fifth Amendment limits on police powers to question, arrest, search, and seize made it impossible for the police to fight crime. Civil liberties advocates heralded the revolution in criminal procedure as a way to ensure that the police treated those accused of crimes fairly.” (https://www.cliffsnotes.com/study-guides/criminal-justice/development-of-the-american-police/policing-the-social-crises-of-the-1960s)
Policing since 1970
One development in policing since 1970 is that the views on domestic violence changed. This was formerly seen as private and a family issue. With the new developments this became a law enforcement issue.
Another development since 1970 was the shift to community oriented policing. This change meant that police were asked to get out of their cars and communicate with the public. The goal was for officers to been seen as members on the community in which the work/ serve. You see some of this with police on foot, bicycles, and horseback. 
References