A 13-year United Nations ban on Iran's ability to buy and sell conventional arms expired on Sunday over the objections of the U.S, which insisted that all UN sanctions on Iran had been reimposed under the "snapback" process of the 2015 nuclear agreement — even though President Trump withdrew from the deal in 2018.

Why it matters: The expiration of the arms embargo will allow Iran to buy advanced weapons systems from countries like Russia and China, upgrading military equipment that dates back to before the 1979 Islamic Revolution, according to the AP.

The big picture: The U.S. tried and failed in August to indefinitely extend the arms embargo, but the UN — including European allies Germany, France and the United Kingdom — dismissed the effort as having no legal basis.

• In response, Trump signed an executive order would impose sanctions on any person or entity that contributes to the transfer of conventional arms to or from Iran or is engaged in providing training and financial support related to those weapons.
• The Trump administration hopes that the wide-ranging sanctions will discourage governments and private companies from buying or selling arms to Iran in fear they will be sanctioned by the U.S. government.

Between the lines: Iran, whose economy has largely been crippled by the Trump administration's campaign of "maximum pressure" sanctions, insisted that it has no intention of going on a weapons "buying spree," which would likely subject other countries to U.S. retaliation.

• The Islamic Republic's military has long been outmatched by regional rivals Saudi Arabia and the United Arab Emirates, which have purchased billions of dollars worth of advanced U.S. arms.
• Tensions between the U.S. and Iran have continued to run high since the January killing of Iranian Gen. Qassem Soleimani. The U.S. has warned Iraqi leaders that it will shutter its embassy in Baghdad if frequent rocket attacks by Iranian-backed militias do not subside.

What they're saying: "Today's normalization of Iran’s defense cooperation with the world is a win for the cause of multilateralism and peace and security in our region," Iranian foreign minister Mohammad Javad Zarif wrote on Twitter.