Were there standards in place for the protection of the President that were relaxed on the day of his assassination, or was it just a case of the assassin getting through the barriers as they existed at that point in time?
Good questions. Presidential protection was very lax in those days. Indeed, the Secret Service was created in 1865 to deal with counterfeiters and only began to protect the president in 1901 after McKinley was assassinated.
Lincoln did have a guard -- four policemen rotated as presidential bodyguards. Three of them had very good records. It was the other guy who was on duty that night. More here.
That’s an interesting link you posted there. After that I read the story of the assassination on wikipedia. Lincoln had several dreams of being assassinated in the days leading up to the event, and spoke of those dreams. One of the people he spoke to about them said “don’t go to the theater tonight” but he already promised his wife he would go.
I bet his dreams were based on reports he was reading of plans on the part of some southern officers to continue fighting after the war ended, threats of guerilla warfare.
Assassination is the ultimate guerrilla warfare.
You have to wonder how different this country would be right now if Lincoln had survived.