What if Germany didn't declare war on the U.S.?
Was a victory still possible for Germany?
My personal opinion is YES.
The Battle of Stalingrad and both Battles of El Alemain (First & Second) were both major turning points of the war, and both were achieved without direct support from the US.
Despite retreating after loosing both battles, Germany, to a certain degree, could have recovered.
In the East....
After the Battle of Stalingrad, Germany was pushed almost entirely out of the Caucasus fields and even lost the city of Kursk.
In Africa....
After the Second Battle of El Alemain, Axis forces were pushed all the way into Tunisia
Here's my What If
US not entering the war in Europe would mean that Great Britain would be fighting the Kriegsmarine alone. The Royal Navy was strong, but Germany's u-boat force was very powerful and was not to be underestimated. Without the US Navy, the RN would struggle with effectively defending their convoys as well as strengthening anti-submarine operations with air power. The Kriegsmarine may have even gotten an upperhand.
Despite no direct support, Great Britain (as well as Russia) would have been supported through the Lend-Lease Act and possibly additional acts. The US would have provided Great Britain with a large quantity of ships, aircraft, materials, etc... to fight off the Kriegsmarine.
In Africa....
Without the US, Great Britain wouldn't have launched Operation Torch; fighting the Axis from Egypt was more than enough. Instead of launching a counter-offensive against the inexperienced American forces, Rommel may have instead attacked the British 8th Army in the East. The Afrika Corps may not have won, but they might have held out as late as mid-late 1943 (Aug-Oct).
If North Africa falls, Great Britain must face launching an amphibious assault on Fortress Europe. Sicily is the most likely target. At this stage in the war, Great Britain was suffering from a very large shortage of manpower. An amphibious assault such as Operation Husky without the US would have been very difficult (yet still possible).
If, and only if, the British achieved success in Sicily, they would have either attacked the Italian mainland or Greece; an assault on France would have been very unlikely. FDR was the one who wanted to attack Germany through France (the better option); Churchill wanted to attack through the Balkans in order to save Eastern Europe from the Soviets and to continue major operations in the Mediterranean. Germany was in a much better position to fight in the Balkans than in France and even Italy. Exposing the Romanian Oil-fields was too much of a risk for Hitler and he wouldn't allow the British to make that much progress in the Balkans. An example of very good German defense in the Balkans was the Dodecanese Campaign.
What if Great Britain does decide to launch an attack on Normandy. Without the US, the British might have succeeded getting a foothold on the beaches. However, without sufficient airpower or ground forces, they might have not withstood a German counter-offensive. The other possibility is that the British wouldn't have had a major breakthrough in Normandy.
In the East....
1943 in the Eastern Front might have nearly been the same as it was historically, despite the US not entering the war. Hitler might have still approved Operation Citadel and Germany could have still lost the Battle of Kursk. The only significant difference that I can think of would be a stronger German defense against the Russian offensives in the East such as a stronger defensive line on the Dnieper River. Another large difference in the East would be the fighting throughout the major Russian summer offensive of 1944, Operation Bagration, mostly because there wouldn't be a 2nd front in the West.
However, for Germany to effectively hold off the Soviets from invading Eastern Europe, the Balkans, and Western Poland, Hitler cannot enforce his "no retreat," and he must allow leaders such as Manstein to properly command the German retreat and any German counter-offensives.
Despite all of this, I think Germany could avoid being conquered by the Soviets. Ultimately it may have been a long war between two major superpowers with Great Britain fighting on the side. The USSR would have obviously had the strategic and numerical advantage, but they may have never had the necessary strength and logistics to fully overwhelm the Wehrmacht, which would still have excelled at a tactical level and would have introduced rocket, jet, and other advanced technologies through time.
If Japan were to launch their attack on Pearl Harbor (very likely), the U.S. would have responded and defeated Japan as they did historically. For this reason, I think that the US would have eventually entered the war in Europe, but in a much slower and less impactful manner.