The American Plan to Blockade Iran
In order to keep Israel from attacking Iran's nuclear weapon sites now, President Obama has made a deal with Israel and promise to create an air and naval blockade of Iran sometime in late summer should the nuclear talks fail.
Iran, starting in July, will face the worse of the sanctions when the oil embargo begins. They will be unable to sell its oil on the world market. If this fails, the next step in late August, an embargo on aircraft and sea vessels visiting Iranian ports and any national airline or international aircraft touching down in Iran will be barred from US and West European airports. The same rule will apply to private and government-owned vessels, including oil tankers. Calling in at an Iranian port will automatically exclude them from entry to a US or European harbor.
How this would effectively work will be interesting. The US 5th Fleet is in the Persian Gulf and it seems this could easily ignite a war because Iran has its own navy, mines, and anti-ship missiles along the shoreline. It is impossible to expect Iran would not use some of this capability.
Of all the sanctions, this US blockade is the most provocative and will no doubt be seen as an act of war. Blockades have been tried in the past and usually end up not working. This plan has been allowed to leak out to the Iranians to apply more pressure at the negotiating table.