> For a transit visa, that makes sense, but you can of course get a
> visitor visa for a country, e.g., Germany without a visa from some
> other country.
If you travel into 'Schengenland' you have to provide proof that you can enter the country you are planning to go to afterwards (you are 'in transit' anytime you travel to a third country after initial entry).
Note a list of the requirements on the german consulate website if you apply from within the US:
http://www.germany-info.org/relaunch/info/consular_services/visa/tourist.html
Required Documents
....
- original of valid US alien registration card or valid US resident visa (type A, E, F, G, H, I, J, L, O, R) and one copy
(The UK is not a party to the Schengen treaty, but it is entirely possible that they want a valid US visa before they admit you for a tourist stay)