Yes, it is true that Marx and Engels identified with the consolidation of the German state under Prussian leadership, and took a dim view of indigenous societies, favouring, for example, the British over the highland tribes (but then, as my aunt still does, so did most lowland Scots).
In an appendix in his Marx and Anglo-Russian Relations, David Riazonov reprints some of Marx's lost articles which are scathing about the 'balance of power' theory of international relations, arguing instead that war is an engine of progress, sweeping aside ossified social relations.
However, there is an important qualification to make, which makes sense of the later views of Lenin and others on anti-militarism. Marx and Engels changed their view of the small nation's war against the big, because of their experience of the influence of the Fenian campaign against Britain. The Irish struggle against British imperialism divided the British workling class between those who were loyal to Britain, and those who sympathised with the Irish rebels. Marx and Engels were of the latter, thinking that this nascent anti-Imperialist struggle was 'a marvellous thing - at once violent and anti British' (from memory).
The change that happened was the growth of what was called in the later nineteenth century, the 'New Imperialism'. Just as middle class liberals were changing their views from being anti-war (like John Bright) to being for the growth of Empire (like Chamerlain and Dilke), Marx and Engels went the other way. They had thought of the liberals' anti-war position as being namby-pamby distaste for social turmoil, but then when the campaign to colonise the developing world hardened up, Marx and Engels switched sides and started to mobilise against the new war drive.
Militarism in the western world, which might still have had a democratic edge to it in the U.S. civil war, or the defence of Paris in 1870, was after that point largely a reactionary influence.
Thereafter, Marx and his followers shifted their position, and organised protests in favour of the Fenians, and against the relief of Omdurman (which Engels' comrade William Morris organised). Their sympathies shifted from being dismissive of 'backward peoples' to being in favour of small nations' struggles against imperialism. Their argument would be that the facts had changed, so they changed their minds.