Essentially, there was a huge debate around this and also a huge sense of betrayal for many in the International. It was, for many of the most militant revolutionaries, a key indicator of the failure of European theorists as all talk, no walk. If you want to see evidence of this, simply look at basically any Bolshevik text from the start of WWI, especially those of Trotsky. There was this real kind of watershed moment where many militants became very disillusioned with the chances of international revolution in that context, though this did absolutely fluctuate throughout the war and in different nations. Basically, the choice was seen as supposed revolutionaries choosing nationalist imperialism over internationalist socialism.

Now, were Bolsheviks such as Lenin surprised at the turn, per se? It's hard to judge. A lot of Marxist writings from the era, especially by Bolsheviks, was very vitriolic - these were intellectuals (some became revolutionaries) throwing theoretical sh*t at each other, and consequently it can be hard to discern the core arguments and sentiments from each other and from hyperbole etc. I would say that, generally, though, Bolsheviks weren't necessarily surprised, but more angry and disappointed. Don't forget, Lenin had already led the Russian Social-Democrat Party into a split in 1902/3 (between the Bolsheviks and Mensheviks) and was an experienced committed revolutionary theorist and activist. Internal divides weren't just normal to him, but were a constant in his life. Therefore, to have another divide may have been frustrating, upsetting, and exasperating, but I don't know if surprise would have been the dominant emotion.

Did he foresee it? Well, lots of socialists saw the dangers of international imperialism in general terms. There was a wide understanding that the exploitation and dominance of the globe was intensifying ever further, and that this created huge tensions. What many socialists hoped was that this would lead to international revolution, especially if there was the threat of international war. But, then it didn't do that, and that was certainly a disappointment. But at the same time, few really understood what WWI was as it started and developed, so few could really see what a huge moment and event (and opportunity for revolution) that it was. By the time many began to return to internationalism, the war was so far progressed that to stop fighting would seem too much of a risk to their nation. This was one of the reason's that the Bolsheviks stance to end the fighting with Germany in 1917 was so shocking to many, and a pivotal moment.

Okay, so I may have missed some specific source details here, but the general sense was no one really saw WWI coming as it did, but there were some ideas floating around. The Bolsheviks were experienced in political combat with their fellow socialists, and thus the divides were large, but not necessarily shocking.