Is INSERT FROM considered a transaction?

Greetings,

Sorry about all the posts lately, but things seems to be running *really*
slow on my database. I have two tables, both are identical and one is used
to hold entries older than a certain date, i.e. the history table. I use
this query to move the old records from one to the other. In this case, is
each insert part of a big transaction that commits when it is done, or is
each insert its own transaction? Is there anything I can do to make this
faster? On average the entries table has about 50,000 records and the
history_entries table has about 3.5 million.

insert into history_entries
select * from entries where domain='somevalue' and time_stamp between
'date1' and 'date2'

Thanks,
Matthew