The announcement by Iran that it has for the first time produced enriched uranium is significant in two ways.
Iran's uranium enrichment was carried out at the Natanz plant
First, it shows that Iran has the technical ability to take this essential step along the path to making nuclear fuel - a path that could also take it to the point where it could make a nuclear bomb, though it says that is not its intention.
Indeed, it says that it enriched only to 3.5%, the level needed for nuclear fuel and way below that needed to make a nuclear bomb, about 80%. Its announcement comes much earlier than many experts had expected, though.
Second, it raises the stakes in the dangerous diplomatic game that is now under way.
Iran is defying demands made by the UN Security Council in March to suspend its enrichment activities while talks take place about the future and the United States is growing increasingly concerned about Iran's long-term intentions.
Iran's defiance and determination is shown by the fact that it made its announcement on the eve of a visit by Mohamed ElBaradei, the director general of the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA).
Mr ElBaradei is making his visit in order to check Iranian compliance with the Security Council demand. He now knows what the Iranian answer is.
So what happens next?
Mr ElBaradei will report back to the Security Council around the end of this month. He will presumably have to say that Iran has failed to suspend its enrichment as called for in the council statement last month.
Iran's announcement was timed to coincide with ElBaradei's visit
The council said that a suspension was necessary to help restore confidence in Iran's intentions, which were severely shaken by the discovery in 2002 of a secret enrichment programme going back some 18 years.
The council will then have to consider its next move. There will be those like the US, the UK and France who will call for the threat of or the application of sanctions.
However, Russia and China, which both hold vetoes, are against sanctions, partly because they do not think they will be effective (Iran seems to thrive on confrontation) and partly because of their own relations with Iran.
If the council is inactive, the European Union is likely to consider joining the United States in imposing unilateral measures. The US has for long imposed trading restrictions on Iran, especially against investment in its oil industry, and formalised them in an Act of Congress in 1996.
However, the EU is unlikely go that far.
Its foreign policy chief, Javier Solana, has drawn up a list of possible measures, including travel restrictions on Iranian officials involved in the nuclear work, restrictions on dual-use technology that could have military applications, a ban on Iranian students studying certain sciences at EU universities and possibly a ban on export credits for some companies trading with Iran.
All that, though, has yet to be decided.
In the meantime, Iran will presumably carry on with enrichment, though there is a super-optimistic view around that it can now afford to compromise, having successfuly, in its view, joined the nuclear club.
Iran's president is embroiled in a dangerous diplomatic game
So far it has used only a cascade of 164 centrifuges to produce small amounts of enriched uranium. This is obtained by spinning uranium gas until the key parts have been separated.
The process is stopped at a certain point for nuclear power fuel but goes on if nuclear bomb material is sought.
Estimates vary as to how long Iran might take to produce enough highly enriched uranium (HEU) to make a nuclear device, if and it is a big if, it decided to do so.
The International Institute for Strategic Studies in London suggested last September that it might be able to get enough HEU for a single bomb "by the end of the decade."
That estimate could come down to about three years if the Iranians were techically proficient but could stretch out to ten to fifteen years if they decided to delay or faced problems. For example they might delay until they construct the large centrifuge plant planned next to the pilot one at Natanz now working. They could then wait and produce a lot of HEU very quickly.
In January the Institute for Science and International Security in Washington said that "Iran could have its first nuclear weapon in 2009", but again that depended on many factors.
There is already talk of the US planning a military strike against Iran, even if at this stage it appears to be contingency thinking only.
Iran says it is simply carrying out the rights it has under the Nuclear Non-Proliferation Treaty.
Iran is correct in claiming that right but the treaty also requires a country to submit to inspections. Iran is complying with a regular inspection regime, though the IAEA is still not satisfied that it has had access to all the information it requires, but is refusing to apply extra inspections it agreed with the IAEA some time ago. So at the very least, the issue of inspections remains unresolved and a resolution is one of the Security Council demands.
Nobody knows the Iranian intentions. It may be that they do not know themselves but want to give themselves the option of becoming a nuclear armed power one day by getting to grips today with a technology they can justify as necessary for nuclear power.