If the Anglo-French fight over Czechoslovakia in 1938, just how easy would it be for the USSR to create a corridor thru Poland or Romania so that it could move its troops to help Czechoslovakia? The goal would be to create this corridor as narrow as possible in order to avoid seeming threatening while also allowing Soviet troops and supplies to easily move through this corridor in large amounts/numbers. And obviously the intention here would be to demonstrate that the Soviet Union will withdraw from this corridor after the end of this war.
Based on the map above, it seems like a Soviet occupation of southeastern Poland would be the best way to go about this since that would secure the major railroad running between the USSR and Czechoslovakia and which runs through Lviv, which was a part of Poland back then. But if the USSR does this, do the Nazis subsequently decide to occupy the rest of Poland? What do you think?
It would be very difficult. Romania would serve as a corridor to fly in supplies and equipment, but Hungary would probably react negatively since they'd likely join in on an invasion to get all of Slovakia back. Poland too would be interested in spoils, but would be more interested in keeping the Soviets off their back more. The only option for a land corridor is through Poland, which means if the Soviets invade Poland is officially a German ally and makes the position of the Allies internationally very difficult, as now the Soviets have invaded a much larger country to save small Czechoslovakia. Poland also isn't a push over vs. the Soviets if they can go all in on the fight in the East, even if the Belarussian and Ukrainian minorities might favor the Soviets. The Soviets in 1938 are even less prepared for war than they were to invade Finland in 1939, so it would be a bloodbath for them, especially if the Germans decide to help the Poles, which they most likely would given that the Poles would keep the Soviets away from Germany. If Poland were collapsing then yes you could see Germany annex west Poland, but at this point Germany and Poland are still very friendly and anti-Soviet together.
BTW these maps are better to see the routes and rail line quality not to mention terrain:
Map Maker: Rand McNallyPublisher: P.F. Collier & SonType(s): World Region Map, Continent MapSize: 11.25" wide x 14.25" highID: 2022041137Collection: WardMaps LLCAtlas: Collier's World Atlas 1937
wardmapsgifts.com
openseadragon.github.io
Likely due to the pace of events the Soviets would get into the war before they were ready and face the million man Polish army, which could get ready much quicker and more effectively due to the greater infrastructure development and smaller size of the country. Due to the motivation differential and Polish innocence (in terms of international support) at this point if the Soviets invade then they'd probably well outperform the Soviets due to greater efficiency and help from nearby countries. The Czechs would get screwed, as the Soviet aggression would overshadow the German aggression against the Czechs, so the Allies might well back down to let Hitler then support Poland against Stalin, since the big fear in the west at this time was another Soviet attempt to invade Europe rather than Hitler. IOTL even in 1940 the French and British were more interested in ways to attack the Soviets than directly fight Germany, hence the effort to get involved in Finland and Operation Pike, not to mention French plans to invade the Balkans.
Ultimately everything will come down to whomever has more staying power. If the Allies cut bait after Czechoslovakia falls (extremely likely), which won't take long for a variety of factors, largely due to how unprepared they were, then it is very hard to see the Soviets keeping the war going long, as getting into a long war with Poland which would likely have international or at least German support would be crippling given the state of war preparedness. This would be a disaster for Stalin, as Poland would then look to Hitler for an alliance, which prevents the OTL start of WW2, and ensures Central Europe largely falls under German sway, probably along with the Balkans. I doubt the Romanians then would do much besides increasingly look to Germany now that both Poland and Germany are effectively allies, since the Allies would not be viewed as particularly valuable after Czechoslovakia falls. The Soviets meanwhile would also be humiliated by how things play out in Poland, especially if they have to then back down as a result of being abandoned by the Allies and being smashed initially by Poland.
Hitler and Poland then get isolated from the west even if peace is made quickly, the Allies rearm, and Stalin also keeps on expanding his forces, but perhaps has more purging to do first. Hitler gets his formal Polish ally and Czechoslovakia in one swoop, Hungary is a confirmed dependency, Mussolini probably increasingly formalizes his relationship to Hitler since he looks strong in the aftermath of the war, and Europe in general looks a lot less stable and waiting for the match to touch of WW2.