Political Memes

The West chose to make the "fight for your survival" play look more viable. If other countries send enough tanks/planes/missiles, perhaps Russia can be pushed back in a matter of a few months without huge loss of territory.

Conversely, if Ukraine was left to their own military and financial resources-- no sanctions to hamstring Russia, no sweetheart deals on equipment-- they could spend a few weeks burning through what they had, and then perhaps degrading to a years-long insurgency situation akin to Afghanistan. The best story you can sell is "We're going to have years of violence and misery, and if we're really lucky, our occupiers will decide we're too much hassle and expense and leave on their own accord." With that alternative, maybe a brokered deal would look more compelling.