Chapter 13 on ‘War as an aid to the progress of the socialist movement?’ explains that, while we are a Marxist party, we don’t necessarily agree with Marx and Engels on everything and in particular not on this point.
“While we are at variance with Marx on the particular question we have been discussing, this does not affect our general acceptance of Marx’s social analysis and his theory of the class struggle. We are a Marxist party but we recognise that the conditions of the time, when Capitalism was relatively young and Feudalism had not yet been completely swept away, led Marx and Engels into a false position on war in the course of pursuing their pioneer work.”