Why did the defeat of Germany seem inevitable by early 1945?
By January of 1945, Germany was in an impossible position militarily. On the Eastern Front, a massive Soviet Army was coming ever closer to the German border, having retaken all of the territory lost earlier in the war, as well as most of Eastern Europe.
On the Western Front, Hitler's last gamble had just failed. He had launched an offensive through the Ardennes Mountains in December 1944 hoping to throw the Allies off balance enough to strike a peace deal. He used the last of his manpower reserves and his best panzer (tank) units, and failed miserably. Germany now faced an Allied army of 2 million soldiers and had very little to defend itself with.
The Luftwaffe, Germany's air force, was a shadow of its former self, and could do little to interfere with the massive B-17 bombing raids hitting German cities night after night. German U-Boats were suffering devastating losses in the Atlantic while an avalanche of men and war material made it safely to French and British ports.
Adolf Hitler himself, in poor health and suffering from tremors and hearing loss related to the assassination attempt against him the previous July, moved into his underground bunker in Berlin, and would not leave there until his death in April.
Germany's defeat didn't just seem inevitable, it clearly was inevitable. They were doomed by 1945, and probably earlier.
I assume that you mean before the D-Day invasion in June of that year.
The reason that the defeat of Germany seemed inevitable was that the Germans were on the defensive all over Europe. They had no chance of invading England anymore. Their invasion of Russia had failed and they were being pushed back on that front as well. The Allies had invaded Sicily and even the Italian mainland.
In addition, the war was going very badly for the Japanese so they could not be expected to help Germany in any way.
The defeat of Germany appeared in inevitable by the beginning of 1945 because of the reverses suffered by the German forces on all the war fronts. For example Germans surrendered at Stalingrad in February 1943, in North Africa in may 1943. Then Allied forces invaded Sicily in July 1943 and landed forces at Salerno Italy in September 1943. By this time Italy surrendered secretly to Allied countries.
The biggest setback to Germany was the landing of allied forces at Normandy on June 6, 1944, followed by landing of about one million allied troops there by end of the month. These troops were advanced across France towards Germany. The results on the battle lines made it very clear that Germany was not left with much capability of resist such assault by Allied forces. Simultaneous pressure was put by troops advancing toward Germany from the Soviet Russia side also, and Germany was not very effective in stopping their advance also. Thus by the beginning of 1945 the defeat of Germany became quite imminent.