US allies fear deployment would spark a Chinese weapons build-up. That could lead India and Pakistan to increase their arsenals, creating a new arms race in Asia. Australia is one of America's few major allies to respond positively to Washington's missile defence plan. Politically, Mr Clinton's decision brings into focus the defence policy differences between Republican presidential nominee George W. Bush and Democratic candidate Al Gore. One will face the issue early in his administration.
Defence is traditionally a Republican issue.
Mr Bush considers the ABM treaty a Cold War relic, unsuited to an era when threats to America come not from Russia but from unpredictable nations. Since the Republican convention in early August, Mr Bush and his vice-presidential running mate, former defence secretary Dick Cheney, have made defence a key issue. It is considered a curious emphasis on a cause not vital to most Americans at a time of peace and prosperity. If Mr Gore were elected in November, he would follow the current administration's cautious position on missile defence. He said after Mr Clinton's announcement that he supported a defence shield if he was fully convinced it was technologically feasible.