Brussels — As British warships steamed for the Falkland Islands, West European governments last week were closing ranks behind Britain with uncharacteristic unanimity and speed.
Only hours after Argentine troops landed on the islands, the 10 European Community countries jointly condemned the ''flagrant violation'' of international law.
And in a weeklong flurry of diplomatic activity unmatched even at the height of the Polish crisis, ambassadors were recalled, arms shipments to Argentina halted, and imports worth nearly $2 billion a year to the Argentine economy banned.
Few diplomats could recall when the EC had pulled together so resolutely for a cause - and an ally.
Early last week, British Prime Minister Margaret Thatcher appealed personally to several European leaders, including French President Francois Mitterrand and West German Chancellor Helmut Schmidt, to take measures ''similar'' to those Britain adopted against Argentina immediately after the Argentine invasion.
But when the EC agreed April 10 to impose a complete ban on Argentine imports , meeting Britain's request to the letter, even seasoned diplomats were surprised. They recall the nit-picking and foot-dragging within the EC following the Soviet invasion of Afghanistan in December 1979 and the military crackdown in Poland four months ago. The action against Argentina is the Community's toughest collective economic sanction ever.
As punishment for the Soviet role in the imposition of martial law in Poland, the diplomats note, the EC wound up cutting Soviet imports by an almost laughable 1.5 percent.
Several times last week the EC called for the immediate application of UN Resolution 502, which demands the withdrawal of Argentine forces from the Falkland Islands and the resolution of the crisis by diplomatic means.
The unanimous and unequivocal condemnation of Argentina (backed by diplomatic and economic pressure) by Britain's European partners contrasts with the Reagan administration's ''neutral'' stance in the crisis so far, European diplomats find it convenient to point out.
But then, according to other officials, the Atlantic Ocean has been growing wider since President Reagan took office 15 months ago.
The Falklands crisis has highlighted the difference in viewpoint between the US and Europe over Latin America and the Caribbean. While the US needs Argentine support for its emerging policy in the region, many countries in Western Europe strongly contest the role the US is seeking to play.
Concerning East-West relations, the reasons Europe was willing to hit Argentina harder for what it did than the Soviet and the Polish military authorities for what they have done are more practical than ideological.
For a start, the Falkland Islands are 8,000 miles away. Only a fence separates West Europe from the Warsaw Pact.
Trade between the EC and Argentina is concentrated in products already overabundant in Western Europe--agricultural goods, footwear, and textiles. A ban on Argentine imports will not hurt the man in the street.
On the other hand, a significant cutback on imports from the Soviet Union--primarily raw materials and energy--could slow a sluggish European economy dramatically.
Finally, several European countries--notably France and Greece--have islands of their own that might be threatened should Argentina be allowed to set a precedent.
How far the Europeans are prepared to go in support of Britain remains to be seen. There has been no talk yet of any country offering military assistance--although NATO planners here in Brussels have taken steps to adjust for the loss of British forces from the North Atlantic.
There is some fear that Britain may be tempted to go too far with gunboat diplomacy. Some European analysts have begun to wonder about the long-term implications of the crisis on Latin America's relations with Moscow, which in recent days has stepped up its criticism of the British action.
But for now, the European plan is to apply economic and diplomatic pressure on Argentina with the aim of forcing it to the negotiating table instead of to war.
''There is no alternative in the present circumstances,'' one European diplomat said.