alternatehistory.com

IOTL the Red Army encircled Budapest in late December 1944, and through February of 1945 besieged it. The Siege tied down Soviet forces for several months, allowed Germany to shore up defenses which were on the verge of collapse, and in the long run delayed the Soviet advance into Austria until March and April.

In November 1944 3rd Ukrainian Front had the opportunity to take the eastern (Pest) portion of the city off the march, with only weak German and Hungarian forces screening the city. They failed, however, and the city was only encircled a month later. This delay gave German and Hungarian forces vital time to reinforce the city and assemble new forces in the region.

So with the delay caused by the siege of Budapest averted, what happens next?
Top