I am not sure that anyone expected that the United States would intervene militarily in the Spanish Civil War. While the US had proven quite willing to intervene in its own ‘backyard’ (ie the Western Hemisphere) leading up to this period, there was ingrained resistance to getting entangled in European conflicts – the difficulty Roosevelt had in persuading Americans to get more involved in the Second World War in 1939-41 is ample demonstration of this way of thinking. While the US was not officially part of the Non-Intervention Committee that sought to control foreign involvement in the conflict, the US Government was quite clear from the beginning that it did not intend to get involved, nor would it allow arms and munitions to be exported to Spain. This was formalised by a House resolution forbidding the export of arms to Spain in January 1937.

Given American unwillingness to become embroiled in European affairs more broadly, it shouldn’t really be a surprise that the government decided to stay out of Spain, especially given that the Spanish Republicans were widely viewed as ‘Reds’, far too closely tied to international communism and revolution to be unconditionally embraced. That said, administration officials, including FDR, did have some sympathy for the Loyalist side that strengthened over the course of the war, and allowed the export of non-military goods such as food (which the Republic desperately needed). While historiography has traditionally viewed American policy as hostile and detrimental to the Republic (‘Malevolent Neutrality’, in the words of Douglas Little), Dominic Tierney has argued that FDR’s growing sympathy for the Republicans, and his increasing efforts to aid them over the course of the war, marks an important phase on FDR’s evolution towards embracing an anti-fascist, interventionist foreign policy. However, American industrial sympathies often went the other way – automakers sold thousands of vehicles to the Nationalists, while the Texas Oil Company allowed them to buy oil on credit. This kind of division was typical of the problems raised by Spain in western democracies – sympathies were split by the conflict, and favouring one side or the other would have been politically very difficult for any government, let alone in such an isolationist country.

With regards to your side question, I’m not certain exactly what you mean – do you mean what other countries ‘sent’ volunteers, or what countries intervened militarily? If the former, Republican volunteers came from many countries (about 35,000 from 50-60 countries by most accounts), but numerically were mostly European, with French, Germans and Italians most common. Smaller numbers of foreign volunteers fought on the other side, mostly from Ireland and France but with scattered other participants. If the latter, countries that sent their own troops include Italy (by far the largest number of regular soldiers, up to 50,000 at any one time, including land, air and sea units), Germany (smaller numbers of specialist units, most famously aircraft but also tanks, artillery and instructors) and the USSR (even smaller numbers of specialists and trainers). Portugal sent a semi-official contingent of volunteers, about 10,000 in total. Although other countries provided aid to one side or another, I don’t think any others actively sent troops to Spain.