Look at Castro now -- cockier than ever with arms and agents to threaten the Americas.

How can the United States act?

Blockade is one answer offered by experts. In it they see a way to isolate Cuba, stop infiltration, maybe finish Castro, too.

This is the question now facing President Kennedy: How to put a stop to the Soviet buildup in Cuba and to Communist infiltration of this hemisphere?

On April 25, the White House reported that a total embargo of remaining U. S. trade with Cuba was being considered. Its aim: To undermine further Cuba's economy. weaken Castro.

Another strategy -- bolder and tougher -- was also attracting notice in Washington: a naval and air blockade to cut Cuba off from the world, destroy Castro.

Blockade, in the view of military and civilian experts, could restore teeth to the Monroe Doctrine. It could halt a flood of Communist arms and strategic supplies now reaching Castro. It could stop Cuban re export of guns and propaganda materials to South America. It would be the most severe reprisal, short of declared war, that the United States could invoke against Castro.

It is the strategy of blockade, therefore, that is suddenly at the center of attention of administration officials, Members of Congress, officers in the Pentagon. As a possible course of action, it also is the center of debate and is raising many questions. Among these questions:

Military experts say a tight naval blockade off Cuban ports and at the approaches to Cuban waters would require two naval task forces, each built around an aircraft carrier with a complement of about 100 planes and several destroyers.