External debt commitments can severely limit the spending capabilities of a global power, particularly in times of total war.
Per the CIA World Factbook, External Debt is the total public and private debt carried by a particular nation as owed to outside parties (i.e. the international community), repayable through currency exchanges, consumer/durable goods and applicable services. Typically, larger, better developed nations will carry a large external debt total. External Debt is just one outside factor used when considering a nation's overall fiscal "health" as going to war tends to have a prominent, far-reaching impact on economies. For the purposes of the final GFP ranking, a high External Debt total serves a penalty.
The data presented below is through 2017. Values presented in $USD.