The bullet points from the first part:
Miscalculations, divisions marked offensive planning by U.S., Ukraine (archived)
Key elements that shaped the counteroffensive and the initial outcome include:
Ukrainian, U.S. and British military officers held eight major tabletop war games to build a campaign plan. But Washington miscalculated the extent to which Ukraine's forces could be transformed into a Western-style fighting force in a short period — especially without giving Kyiv air power integral to modern militaries.
U.S. and Ukrainian officials sharply disagreed at times over strategy, tactics and timing. The Pentagon wanted the assault to begin in mid-April to prevent Russia from continuing to strengthen its lines. The Ukrainians hesitated, insisting they weren't ready without additional weapons and training.
U.S. military officials were confident that a mechanized frontal attack on Russian lines was feasible with the troops and weapons that Ukraine had. The simulations concluded that Kyiv's forces, in the best case, could reach the Sea of Azov and cut off Russian troops in the south in 60 to 90 days.
The United States advocated a focused assault along that southern axis, but Ukraine's leadership believed its forces had to attack at three distinct points along the 600-mile front, southward toward both Melitopol and Berdyansk on the Sea of Azov and east toward the embattled city of Bakhmut.
The U.S. intelligence community had a more downbeat view than the U.S. military, assessing that the offensive had only a 50-50 chance of success given the stout, multilayered defenses Russia had built up over the winter and spring.
Many in Ukraine and the West underestimated Russia's ability to rebound from battlefield disasters and exploit its perennial strengths: manpower, mines and a willingness to sacrifice lives on a scale that few other countries can countenance.
As the expected launch of the offensive approached, Ukrainian military officials feared they would suffer catastrophic losses — while American officials believed the toll would ultimately be higher without a decisive assault.