ACTA has replaced SOPA as the biggest threat to Internet freedom worldwide, advocates say. 

The Anti-Counterfeiting Trade Agreement treaty has come into the spotlight since last week, when the U.S. House of Representatives shelved the Stop Online Piracy Act (SOPA) and the Senate the related Protect IP Act.

Backers say the ACTA treaty, like the SOPA bill, aims to limit counterfeiting and piracy, but opponents argue that both bills would have adverse effects such as quashing innovation, limiting Internet freedom and censoring the Internet.

In response to such concerns, Wikipedia, GoogleReddit and other websites staged a successful SOPA blackout Jan. 18, bringing the Web together against the bill in a protest that led to the congressional retreat.

Now ACTA opponents, from the Electronic Frontier Foundation to the Anonymous hacktivist collective, are colaescing in opposition to the treaty.

Despite their many similarities, opponents believe ACTA is the biggest threat to Internet freedom in terms of its potential worldwide impact. Here are five reasons:

1. Scope: The key reason ACTA is the biggest threat to the Internet as we know it is that it's an international treaty, while SOPA was a bill before the House. Although it would have had some worldwide implications, it was aimed at stopping Web piracy at overseas sites like Sweden's Pirate Bay torrent site.

ACTA, meanwhile, would set up an international legal framework to deal with issues of counterfeiting, piracy and other crimes. Instead of dealing with national laws regarding these issues, these nations would be able to adjudicate alleged crimes in a new governing body that would exist outside of the purview of the United Nations and other international institutions.

This opens up the possibility for ACTA to be used by a coordinated authority that rests outside of any country to crack down on Internet activity throughout the world. SOPA, meanwhile, would have been enforced by existing American agencies, and would have been subject to legal scrutiny and constitutional challenges within the U.S. judiciary system.

2. Transparency: The SOPA debate took place mostly outside of the public eye at first, but because it was taking place in the halls of Congress, Internet advocates were able to follow changes to the legislation, view draft bills and related materials and monitor the proceedings.

But the ACTA treaty is being negotiated almost entirely behind closed doors. If it were not for the advent of WikiLeaks, which released documents revealing details of the negotiations and draft versions of the treaty, the world would still have very little knowledge of what exactly the treaty might entail.

This opens up major concerns for those who would like to ensure the treaty does not infringe on civil rights or limit Internet freedom in ways that would be detrimental to the web's status as a place where information flows freely.

3. Ease of Approval: The SOPA bill was derailed because it required both houses of Congress to pass it.  Once approved, it would have been subject to challenge and could have been changed by future congresses.

ACTA, on the other hand, was already signed by the U.S. on Oct. 11, 2011, and President Obama was not required to get the approval of any outside authority to do so.

Under the U.S. Constitution, the Senate is required to ratifty treaties. At least one critic, Sen. Ron Wyden (D-Ore.), has objected about this, claiming in an Oct. 12 statement that it needs to be brought up for ratification in the Senate:

It may be possible for the U.S. to implement ACTA or any other trade agreement, once validly entered, without legislation if the agreement requires no change in U.S. law, Wyden writes. But regardless of whether the agreement requires changes in U.S. law...the executive branch lacks constitutional authority to enter a binding international agreement covering issues delegated by the Constitution to Congress' authority, absent congressional approval.

Now that it has been signed, the legislative and judicial branches of the U.S. government also have little ability to challenge or amend the treaty, and Americans would be subject to a whole new scheme of laws, restrictions and regulations that could have them facing fines or jail time through a process that would likely exist entirely outside the scope of the American justice system.

4. Level of Support: Even before the efforts of opponents brought down the controversial legislation, SOPA had only 31 co-sponsors in Congress, meaning it was never a wildly-popular bill to begin with. Despite loud cries that SOPA was threatening the Internet and that Congress was about to pass it, it was never close to being made into law.

ACTA, on the other hand, is an international treaty, meaning that it requires unilateral signatures, not votes based at least in part on public opinion. And the Obama administration already signed the treaty.

And the U.S. is not alone in having signed on to ACTA: During the same Oct. 11 signing ceremony in Tokyo, Australia, Canada, Japan, Morocco, New Zealand, Singapore, and South Korea also signed, while the European Union, Mexico, and Switzerland have said they plan to do so in the near future.

The G8 has also come out with a statement in support of ACTA, offering the following remarks on the topic in a 2008 communique: We encourage the acceleration of negotiations to establish a new international legal framework, the Anti-Counterfeiting Trade Agreement (ACTA), and seek to complete the negotiation by the end of this year.

5. Visibility: The campaign to stop SOPA began relatively early on in its development. By the time it was even able to go to mark-up in the House Judiciary Committee, opponents were already loudly making their opinions known to the Internet-using public.

ACTA, on the other hand, is largely off most people's radars, though it has been under official negotiation for about five years. Protest efforts against it in the U.S. are modest at best.  But they do persist, as a petition on with more than 6,000 signatures currently on file with the Obama administration, calling on it to end ACTA and protect our right to Internet privacy.

But ACTA's visibility is rising in nations that have yet to sign the treaty. Poland signed the treaty Thursday, drawing the ire of Polish opponents, including representatives of the Anonymous collective. Anonymous shut down a number of Polish government websites--including that of the Polish prime minister--in order to demonstrate its opposition to ACTA, Poland's reported.