By Laura Smith-Spark
BBC News, Washington
US leader George W Bush has exercised his presidential veto for only the second time in seven years of office.
George W Bush used his veto for the first time on a stem cell bill
The move was prompted by the approval by Congress of a bill linking war funding to a timetable for withdrawal of combat troops from Iraq.
The Democrat-controlled Congress passed the bill narrowly despite Mr Bush's repeated threats to veto it.
Once the bill was presented to Mr Bush, he had 10 days in which to return it to Congress with his objections - but opted instead to wield his veto within hours.
Both the House and Senate would have to muster a two-thirds majority in favour of the bill if they were to override the veto.
Given the Democrats' slim majorities they are highly unlikely to succeed, meaning the legislation will have to be revised and approved in both Houses again before returning to the president.
So how much significance does the veto have - and can we expect to see it wielded more often in future?
Mr Bush has made very little use of what is undoubtedly one of the most powerful tools the president has to rein in Congress.
Highest number exercised:
F D Roosevelt (1933-45) - 372
G Cleveland (1885-89 & 1893-97) - 346
H S Truman (1945-53) - 180
DD Eisenhower (1953-61) - 73
Most recent presidents:
Ronald Reagan (1981-89) - 39
George Bush (1989-93) - 29
Bill Clinton (1993-2001) - 36
George W Bush (2001-) - 2
Figures for regular vetoes only; pocket vetoes are not included
His only previous veto came last year, when he refused to sign into law a controversial bill which would have lifted a ban on federal funding for new embryonic stem cell research.
In waiting until the fifth year of his presidency to do so, Mr Bush became the first president to complete four years in office without a veto since John Quincy Adams in the 1820s.
His predecessors at the White House have made far greater use of the measure.
The biggest veto-er in US history is Franklin D Roosevelt with 635, of which 372 were regular vetoes and 263 so-called pocket vetoes, whereby if Congress adjourns before meeting the president's objections, the bill does not become law.
Grover Cleveland, the 22nd and 24th US president, comes second, having exercised his regular veto 304 times in his first term and 42 in his second.
Former President Bill Clinton used his power of regular veto 36 times, twice to impede bills passed by Congress to ban a late-term abortion procedure. Only two of his vetoes were over-turned by Congress.
The current president's father, George H W Bush, wielded the power of direct veto on 29 occasions and had 15 pocket vetoes.
According to political analyst Larry Sabato, Mr Bush's reluctance to exercise his veto during six years of Republican-controlled Congress, before the Democrats gained sway last year, may have hurt him and his party.
"Other presidents have used the veto hundreds of times," says Mr Sabato, director of the University of Virginia's Center for Politics.
"This is one of the least impressive aspects of the Bush presidency - it suggests weakness and it has been a major mistake by Bush.
"His failure to veto bills during the periods of Republican-controlled Congress meant he allowed spending to get completely out of hand.
"They would have benefited from discipline. That is what the veto is - it's executive discipline applied to the Congress."
On the other hand, Mr Sabato believes Mr Bush's threat to veto the war funding bill played into the Democrats' hands because it allowed them to propose legislation that they knew could never be passed but which was popular with supporters.
There are likely to be more such legislative deadlocks before Mr Bush's presidency comes to an end.
The White House has warned troops will suffer hardship without funds
John Sides, assistant professor of political science at George Washington University, says:
"This is the beginning of a lot of conflict between the Congress and the president.
"He was able to work quite harmoniously with the Republican Congress in the first six years of his term in office, in some ways because the president's agenda was really leading the congressional agenda.
"Leaders in Congress were very conscious of being loyal to him. But when the president's approval rating starts to go down, people start jumping ship - or at least looking longingly overboard."
In the period following the 9/11 terror attacks, the president made broad use of executive powers and faced little opposition in Congress.
As the number of investigations into his administration's handling of affairs multiplies, however, Mr Bush may suffer from a perception that he has not been interested in involving Congress, Mr Sides says.
Now Mr Bush has used his veto, the pressure is on both sides to reach agreement on a new bill before funding for US operations in Iraq and Afghanistan runs out in the summer.
"Ultimately there will be some effort to achieve a compromise of sorts and the question is, who will blink first?" says Mr Sides.
"There's a game of chicken going on and we will see who veers off first."