With the exception of a few minor battles and Yavin and Endor(which was lost due to a combination of the Emperor's restriction on the Imperial Fleet, the loss of chain of command and INCREDIBLE LUCK on the part of the Rebellion) the Empire did pretty much smash the Rebellion at will. The Empire's Fragmentation after Endor is what destroyed them, they really doomed themselves more than the NR ever did. To a degree I believe the Imperial Remnant could have won had it been united early on rather than splitting so much. What doomed the Empire was Warlordism, internal politics and the loss of the Emperor. Isard was a complete power hungry FOOL who arguably caused the most damage to the Empire through her manipulations and so called schemes after palpatine's death.
Following this were the seemingly endless power struggles between the feuding Warlords, Harrsk, Teradoc, Delvardus, Krennal, Drommel, Grunger, Pitta, Kaine, Zsinj and numerous others that whittled the Empire's military down to paltry numbers and territory.
Thrawn showed just what the Empire could do when it was united and focused properly! In less than a year he reconquered half the galaxy and had the NR reeling! Then his death sent everything in a downward spiral again.
The REAL kicker was the Imperial mutiny that utterly destroyed any unity and military prowess Grand Admiral Thrawn had rebuilt.
I feel that the Empire had no real chance to win the war after the last death of Palpatine and the loss of the World Devastators, Galaxy Gun, Eclipse, Eclipse II, the Sovereigns, Shadow Hand Fleet, Byss, all the Emperors most experienced advisers and Dark Jedi.
The loss of the Interrim Ruling council, and more importantly their secrets, as well as the continuing damage done to Imperial forces by the infighting Warlords pretty much sealed the outcome.
By the time Daala and Pellaeon got control it was too little too late. It was only a matter of how long they could postpone the inevitable.