By Gordon Corera
BBC News security correspondent
The alleged plot shows similarities to attacks in London in 2005
German authorities are making clear that they believe they have foiled a major terrorist plot - and add that the country remains in the firing line.
"It has been shown once again that our country is a target of international terrorism and not just a resting or a withdrawal area for the terrorists," said the federal prosecutor at a news conference.
Other European nations will also be watching with some anxiety.
Over the last few months, there had been numerous reports suggesting a threat against Germany and particularly against US targets.
In April, the US embassy in Berlin announced it was increasing security at American facilities in the country.
The men who were arrested appear to have been under surveillance for some time. In late 2006 one individual was seen spying on US barracks, leading to a surveillance operation.
In that operation the group was seen slowly acquiring 12 barrels of hydrogen peroxide - the same substance used in UK attacks in 2005.
The authorities were able to replace the hydrogen peroxide with a weaker, less dangerous solution but were still worried about the risk.
Germany was used as a base for the planning of 9/11 (the so-called "Hamburg cell") but it is only recently that the sense of threat has increased, with failed attacks on trains in 2006 and also concerns over the impact of the presence of German troops in Afghanistan.
In June, three Germans were also arrested in the border region between Afghanistan and Pakistan and the authorities believe that up to a dozen others may have travelled there for training.
The arrested men had been under surveillance for some time
A recent US intelligence report talked of al-Qaeda finding a "safe haven" in Pakistan, and the flow of individuals from Europe to the lawless Pakistan-Afghanistan border region for training before returning home to plan attacks is one of the most serious concerns for security and intelligence officials.
It is the model used by some of those involved in the London bombings of 7 July and attempted attacks of 21 July in 2005, as well as in the German case.
Officials also say that summer onwards is a time of heightened threat because of the cycle of training activity.
Recruits tend to head to the remote border regions before winter sets in and then stay there until spring. After that, they often return home and begin the process of planning attacks.
One fear is the impact of German troops' presence in Afghanistan
Coming the day after arrests in Denmark, the German operation has raised concerns over increased al-Qaeda activity across Europe and whether other cells are active.
So far there is no hard evidence of a direct link between the German and Danish cases, but Danish intelligence officials did say that they believed the group they arrested on Tuesday was linked to senior al-Qaeda figures.
US officials have also compared the current threat environment to the summer of 2001, with talk of "chatter" being intercepted between individuals indicating possible plans of attacks.
With the anniversary of 9/11 approaching, this may be a nervous time for counter-terrorism officials in many countries.