“Having destroyed the social power of the nobility and the guildmasters, the bourgeois also destroyed their political power. Having raised itself to the actual position of first class in society, it proclaims itself to be also the dominant political class. This it does through the introduction of the representative system which rests on bourgeois, and these bourgeois deputies, by using their right to refuse to vote taxes, choose a bourgeois government.”

Source: Reform or Revolution (1899), Ch.8

Order by the commissar for military affairs - on the murder of count Mirbach
How the Revolution Armed (1923)

Source: The Communist Manifesto (1848), Section 2 paragraph 7.

The Junius Pamphlet http://www.marxists.org/archive/luxemburg/1915/junius/index.htm (1915)
Context: Bourgeois class domination is undoubtedly an historical necessity, but, so too, the rising of the working class against it. Capital is an historical necessity, but, so too, its grave digger, the socialist proletariat.

Collected Works, Vol. 31.
Collected Works

1930s, Die verfluchten Hakenkreuzler. Etwas zum Nachdenken (1932)

Principles of Communism (1847)

Source: The Manifesto of the Communist Party (1848), Section 2, paragraph 72 (last paragraph).

No Compromise – No Political Trading (1899)
Context: This foundation of the class struggle, which Marx – and this is his immortal service – has given to the modern labor movement, is the main point of attack in the battle which the bourgeois political economy is waging with socialism. The political economists deny the class struggle and would make of the labor movement only a part of the bourgeois party movements, and the Social Democracy only a division of the bourgeois democracy. The bourgeois political economy and politics direct all their exertions against the class character of the modern labor movement. If it were possible to create a breach in this bulwark, in this citadel of the Social Democracy, then the Social Democracy is conquered, and the proletariat thrown back under the dominion of capitalistic society. However small such a breach may be in the beginning, the enemy has the power to widen it and the certainty of final victory. And the enemy is most dangerous when he comes as a friend to the fortress, when he slinks in under the cover of friendship, and is recognized as a friend and comrade. The enemy who comes to us with open visor we face with a smile; to set our foot upon his neck is mere play for us. The stupidly brutal acts of violence of police politicians, the outrages of anti-socialist laws, the anti-revolution laws, penitentiary bills – these only arouse feelings of pitying contempt; the enemy, however, that reaches out the hand to us for a political alliance; and intrudes himself upon us as a friend and brother, – him and him alone have we to fear. Our fortress can withstand every assault – it can not be stormed nor taken from us by siege – it can only fall when we ourselves open the doors to the enemy and take him into our ranks as a fellow comrade. Growing out of the class struggle, our party rests upon the class struggle as a condition of its existence. Through and with that struggle the party is unconquerable; without it the party is lost, for it will have lost the source of its strength. Whoever fails to understand this or thinks that the class struggle is a dead issue, or that class antagonisms are gradually being effaced, stands upon the basis of bourgeois philosophy.