Preparations for the conflict was obvious for quite some time, yet the Portuguese government did not provide credible military defence and zero air support.  Even worse,  Antonio de Oliveira Salazar's government in Lisbon tried to prevent evacuation of Portuguese civilians despite imminent hostilities.

 

Understandably, Portugal was facing serious unrest in much of her Asian and African colonies, yet Salazar's government seemed to foolishly underestimated the vulnerability of Portuguese position in India by not providing strong military support to Governor General Manuel Vassalo e Silva of Goa for the latter's impossible task.

 

How could the fiasco (from Portugal's perspective) been possible?

Were the collective Portuguese leadership blind to their weakness or were they counting on foreign support in the event of war?

 

Elsewhere, they were able to hold on to their colonies longer and were initially successful in suppressing armed hostilities.

Those conflicts were against poorly equipped and poorly trained guerillas. In India, the Portuguese were facing a large and capable conventional army that inherited alot of modern combat strategies from the British. In addition, many of the Indian officers and NCOS had combat experience against Pakistan (also a capable military force).  Portugal simply did not have anywhere near the military power to stop India in a conventional conflict.

 

A successful defense of Goa would have required the combat power of Great Britain (heavy units defend Goa against conventional offensive, Royal Navy blockades all Indian ports and Royal Airforce / Carrier airpower starts to destroy logistics and other infrastructure targets).  It would have also required access to strategic logistics staging areas (Singapore) that Portugal did not have. Even with power like Britain, a successful defense would have been difficult.