International freight going between Canada, the United States, and Mexico has always been a tricky situation for many drivers. Regardless of what you think about the increased ease of drivers from international locations bringing loads into the United States, it is legal and an increasingly common practice.
Drivers with a current and valid CDL from Canada or Mexico are not required to obtain a CDL within the United States unless they are moving to the United States to live. In that case they would apply for the appropriate state CDL in which they are establishing residence. In some specific situations and based on state laws a nonresident may apply for a USA CDL as a nonresident or nondomicle CDL. Most states put restrictions on these types of nonresident CDLs and typically exclude some endorsements such as hazardous material transport. Texas, for examples, offers nonresidents a temporary, 60 day nonresident CDL that cannot be renewed and cannot include a hazardous material endorsement.
The most important thing that international CDL drivers have to keep in mind, which is the same as US CDL holders driving outside of the USA, is that the rules for licensing and driving are based on the country you are in, not the country that issued your CDL. Canada, Mexico and the United States all have their own set of requirements for CDL but once you have that license it is valid in the United States provided it is not revoked or otherwise invalid in the country of issue.
All drivers entering the United States with commercial cargo have to have an International Carrier Bond. This bond is issued from an approved and certified company that basically insures you as transporter of goods into and through the United States. There is a form that must be completed as well as fees that are paid in advance and then the bond is on file with the border crossing that you use the most often. You can opt to pay a yearly fee for border crossing and simply display a decal in your window to expedite this process.