I can't find that exact quote, but I did find a similar quote from 'The Movements of 1847', an article written by Engels for the Deutsche-Brüsseler-Zeitung, published January 23rd, 1848.

In America we have witnessed the conquest of Mexico and have rejoiced at it. It is also an advance when a country which has hitherto been exclusively wrapped up in its own affairs, perpetually rent with civil wars, and completely hindered in its development, a country whose best prospect had been to become industrially subject to Britain — when such a country is forcibly drawn into the historical process. It is to the interest of its own development that Mexico will in future be placed under the tutelage of the United States. The evolution of the whole of America will profit by the fact that the United States, by the possession of California, obtains command of the Pacific. But again we ask: “Who is going to profit immediately by the war?” The bourgeoisie alone. The North Americans acquire new regions in California and New Mexico for the creation of fresh capital, that is, for calling new bourgeois into being, and enriching those already in existence; for all capital created today flows into the hands of the bourgeoisie. And what about the proposed cut through the Tehuantepec isthmus? Who is likely to gain by that? Who else but the American shipping owners? Rule over the Pacific, who will gain by that but these same shipping owners? The new customers for the products of industry, customers who will come into being in the newly acquired territories — who will supply their needs? None other than the American manufacturers.

Link: https://marxists.architexturez.net/archive/marx/works/1848/01/23.htm