history

If we're talking the Berlin Wall in specific, this passage from chapter 8 of William Blum's Killing Hope: US Military & CIA Interventions since WWII is very relevant.

The Association of Political Refugees from the East, and the Investigating Committee of Freedom-minded Jurists of the Soviet Zone, were two of the other groups involved in the campaign against East Germany. The actions carried out by these operatives ran the spectrum from juvenile delinquency to terrorism; anything "to make the commies look bad". It added up to the following remarkable record:

• through explosives, arson, short circuiting, and other methods they damaged power stations, shipyards, a dam, canals, docks, public buildings, gas stations, shops, a radio station, outdoor stands, public transportation;
• derailed freight trains, seriously injuring workers; burned 12 cars of a freight train and destroyed air pressure hoses of others;
• blew up road and railway bridges; placed explosives on a railway bridge of the Berlin-Moscow line but these were discovered in time—hundreds would have been killed;
• used special acids to damage vital factory machinery; put sand in the turbine of a factory, bringing it to a standstill; set fire to a tile-producing factory; promoted work slow-downs in factories; stole blueprints and samples of new technical developments;
• killed 7,000 cows of a co-operative dairy by poisoning the wax coating of the wire used to bale the cows' corn fodder;
• added soap to powdered milk destined for East German schools;
• raided and wrecked left-wing offices in East and West Berlin, stole membership lists; assaulted and kidnapped leftists and, on occasion, murdered them;
• set off stink bombs to disrupt political meetings
• floated balloons which burst in the air, scattering thousands of propaganda pamphlets down upon East Germans;
• were in possession, when arrested, of a large quantity of the poison cantharidin with which it was planned to produce poisoned cigarettes to kill leading East Germans;
• attempted to disrupt the World Youth Festival in East Berlin by sending out forged invitations, false promises of free bed and board, false notices of cancellations; carried out attacks on participants with explosives, firebombs, and tire-puncturing equipment; set fire to a wooden bridge on a main motorway leading to the festival;
• forged and distributed large quantities of food ration cards—for example, for 60,000 pounds of meat—to cause confusion, shortages and resentment;
• sent out forged tax notices and other government directives and documents to foster disorganization and inefficiency within industry and unions;
• "gave considerable aid and comfort" to East Germans who staged an uprising on 17 June 1953; during and after the uprising, the US radio station in West Berlin, RIAS (Radio In the American Sector), issued inflammatory broadcasts into East Germany appealing to the populace to resist the government; RIAS also broadcast warnings to witnesses in at least one East German criminal case being monitored by the Investigating Committee of Freedom-minded Jurists of the Soviet Zone that they would be added to the committee's files of "accused persons" if they lied.

Although many hundreds of the American agents were caught and tried by East Germany, the ease with which they could pass back and forth between the two sectors and infiltrate different enterprises without any language barrier provided opportunities for the CIA unmatched anywhere else in Eastern Europe.

Throughout the 1950s, the East Germans and the Soviet Union repeatedly lodged complaints with the Soviets' erstwhile allies in the West and with the United Nations about specific sabotage and espionage activities and called for the closure of the offices in West Germany they claimed were responsible, and for which they provided names and addresses. Inevitably the East Germans began to tighten up entry into the country from the West.

The West also bedeviled the East with a vigorous campaign of recruiting East German professionals and skilled workers. Eventually, this led to a severe labor and production crisis in the East, and in August 1961, to the building of the infamous Berlin Wall.

The passage starts on page 61 of the pdf, and corresponding citations are on page 228.