After the defeat in the national revolutionary war against fascism (1936 -39), the political leadership of the PCE did not undertake a rigorous analysis of the causes of the defeat and the role of the Party in the final phase of the war. The party leadership, with Comrade Jose Díaz  seriously ill and being itself dispersed in different countries, failed to articulate a strategy for continuing the war against fascism until the beginning of the Second World War. There was no fallback plan, and even less, a forecast that allowed to continue the organized struggle underground.
From 1932 to 1954 no Congrses of the PCE was held , allowing a constant and progressive weakening of the Leninist principles of collective leadership and an ideal setting for all types of maneuvers made without considering the organicity and the struggling basis and militants of the party. Situation further enhanced by a Political Bureau, whose members lived thousands of miles away from each other and without the presence of an articulate and effective political leadership inside the country.
Parallel to the formulation of the “Italian path to socialism”, the PCE adopts in Spain the so-called “policy of national reconciliation”, while undertaking a disastrous retreat of the guerrilla struggle. With such precedents, a hard battle begins in the leadership of the PCE.
Led by Carrillo, appointed Secretary General at the 6th Congress, held in Prague in December of 1959 and January 1960, the leadership prepares the so-called “democratic way out”, designs the so-called “alliance of labour and culture forces” and progressively imposes a revisionist and anti-Soviet line, eliminating prominent leaders, removing the cadres who, in the party leadership remained loyal to Marxism-Leninism, and expelling thousands of honest communists who heroically fought inside the country.
The Eurocommunist fraction relied all the time on the results of the 20th Congress of the CPSU, especially in the thesis that asserted the plurality of forms in the transition to socialism and the criticism of Stalin contained in the Secret Report, which served as a pretext to defame the USSR and move away from the teachings of the October Revolution in the revolutionary transition and the building of socialism. They also relied for that purpose in the counterrevolutionary events of October-November in the Popular Republic of Hungary and especially in the Warsaw Pact intervention in Czechoslovakia, used together with the above to undermine the confidence of the militants and the working class in socialism and reduce the immense prestige of the USSR.
The opportunism of the Eurocommunist leadership of the PCE knew no bounds. In 1970 Santiago Carrillo said to the French daily Le Monde:
“We conceive a socialist Spain where the Prime Minister would be a Catholic and where the CP would be a minority ... Spanish socialism will march with the sickle and hammer in one hand and the cross on another.” 
Since then, the wording of the so-called “covenant for freedom” comes to the forefront in the PCE. As in the PCI with the “historic compromise”, the above mentioned covenant, the maximum expression of the triumph of interclassism in the PCE, is not conceived as an alliance of classes or political organizations to overcome the dictatorship, but in its Eurocommunist application, it becomes the desperate search for recognition by the ruling classes, especially of the oligarchy that opposed their interests to Franco's autocratic tendency and struggled within the regime for the Spanish integration in the European Economic Community, which at the political level required a change in the form of domination, a protected passage from dictatorship to parliamentary monarchy.
And in this passage the revisionist PCE was committed. First accepting the “Moncloa Agreements” which subjected the interests of the working class and popular sectors to the economic interests of the oligarchy, in the middle of the economic crisis, playing a role of containment of workers' struggle. After that, accepting the monarchy, burying the history of anti-fascist struggle of the working class and the Spanish people, giving up the re-establishment of republican legality and supporting the Constitution of 1978, which consecrated the change from one form to another in the exercise of the dictatorship of capital.
In parallel, from the CC plenary held in 1976 in Rome, the Leninist conception of the Party, its place and the its role in society, its functions and essential tasks, its organizational principles, were attacked. In a party with thousands of purged members, the doors of the party were opened wide to thousands of new members without any control or revolutionary monitoring. All conditions were stablished in order to formally approve, in the 9th Congress, held in Madrid in 1978, the abandonment of Marxism-Leninism and the consecration of the revisionist policy imposed after a long process to the Spanish communists.
The Party of the national revolutionary war, the guerrilla warfare, whose militants formed in the resistance against Nazi-fascism in all European countries and fought without mercy together with the Soviet people in the battles of Leningrad and Stalingrad, had been liquidated.
The PCE had mutated beyond recognition in an organization that, even until today, is against the historical necessity of socialist revolution and the revolutionary power of the working class - the dictatorship of the proletariat - in the transition period and the construction of socialism; a party that is opposed to the Leninist principles of organization, especially to democratic centralism; a party that renounces to the experience and lessons of socialist construction in the twentieth century, which qualifies as a sort of “state capitalism”, rejecting in particular the period known as “socialist attack or assault against capitalism” in which the Soviet Union, with Stalin at the head of the CPSU, demonstrated the superiority of socialism over capitalism and achieved major successes; a party that accepts the imperialist framework of the European Union, claiming for a social and democratic version of the same under the opportunist postulates of the European Left Party; and a party that rejects all forms of recomposition of the international communist movement structured on firm ideological foundations.
In the Iberian Peninsula, the fraternal Portuguese Communist Party withstood all kinds of pressures that, seeing among others the Spanish example, sought to end the Marxist-Leninist line of the PCP. Comrade Alvaro Cunhal, Secretary General of the PCP responded always firmly and decisively:
“This campaign appears frequently with a paternalistic tone. They lament what they call the “inflexibility”, the “dogmatism”, the “sectarianism”, the “Stalinism” of the PCP and do hope that the PCP will become a “modern” and “western” party...
And what are the modifications that the PCP would do to “prove its independence”?
The conditions are pointed provocatively. They all revolve around six major points: stop being a Marxist-Leninist party, breaking the friendly relations with the Communist Party of the Soviet Union, criticizing the Soviet Union and the socialist countries, breaking with proletarian internationalism, abandoning in Portugal the structural reforms of a socialist character and adopting an internal operation that allows trends and divisions and breaking the unity of the Party.” 
In the Spanish communist movement, unlike the Portuguese, the revisionist positions promoted by the leaders of the PCE became hegemonic, and throughout this process the PCE was divided into two main forces: those who resisted the Eurocommunistoffensive and defended Marxism-Leninism grouping in 1984 in the Communist Party of the Peoples of Spain, and those who persisted and persist in wallowing in the revisionist swamp, without having made a serious and rigorous self-criticism, a simple analysis that goes beyond mere lamentations about what the “Spanish transition” could have been but was not and continue to defend in the practice the path of bourgeois parliamentarism wrapped up, nowadays, with the same Republican flag that once they betrayed.
Let us give an example of this. In the organ of expression of the PCE from April 2010, under the title “Political offensive towards the Republican Conference of the PCE”, the Republican Movement Secretary of the PCE says among other niceties:
"In the PCE we understand that the republican project should not be pigeonholed in terms of terminology referring to spaces in the political spectrum. We must give the word Republic an entity of proposal to make it more accessible and appealing; the Republic is the economic, social, political, ideological reform and the reform of new values to the real situation.”
Then, the Director of “Mundo Obrero”, in his article entitled “Building the Republic” gives us even more clear signs of complete confusion within the reformism:
"We are not against the Constitution whose deep reform we are asking for, we are clear that the goal is against an archaic monarchy, obsolete and guarantor of the values of neoliberalism. We do not want any republic, but a federal and democratic one with the values of the 1st and 2nd Republics applied to the current situation...
The future republican Constitution should be focused in the contents of the solemn declaration of the UN Human Rights from December 10, 1948, and must also adopt the three covenants signed in 1966 and accepted by Spain which develop those contents...
Democracy as a permanent agreement between free and equal beings to keep agreeing permanently has a range and depth that enables the public accessibility to making all kinds of decisions ...”
The old revisionist content, adopted in Spain and other countries as “Eurocommunist”, thus fits with the times. New language for old approaches and no trace of Marxism. The theses of the 18thCongress of the PCE say:
“At this 18th Congress, the PCE is reaffirmed in the defense of socialism as a coherent development and full implementation of democracy. Therefore it includes the recognition of the value of personal freedoms and their guarantee, the principles of secular state and its democratic articulation, the plurality of parties, trade union autonomy, freedom of religion and worship practiced in the private sphere and the total freedom of inquiry, and artistic and cultural activities.”
Exactly the same as the Eurocommunist PCE said after the Central Committee plenum held in Rome in 1976, whose quote we have reproduced above.
The so-called Socialism of the 21st Century is the new flag of our present republicans and yesterday Eurocommunists.  A proposal whose most elaborated versions depart from these revisionist theses that have crossed the central debates of the labour movement since it entered in History, from Bernstein to Eurocommunism, opposing to scientific socialism an exercise of eclecticism mixed with liberal – bourgeois positions.
Therefore it is not surprising that the parties heirs of Eurocommunism have warmly greeted the proposal of a 5th International , where their revisionist approaches can coexist naturally with forces that have fully renounced to the class struggle, with all kinds of social democrats, Trotskyists and every modern variety of opportunism, both right and left, as they already do at a regional level in the European Left Party.